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* Re: [PATCH v6 4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]
       [not found]   ` <87v90xryvi.fsf@igel.home>
@ 2022-09-11 20:12     ` Sunil Pandey
  2022-09-11 20:15       ` Arjan van de Ven
  2022-09-29  0:09       ` Noah Goldstein
  0 siblings, 2 replies; 8+ messages in thread
From: Sunil Pandey @ 2022-09-11 20:12 UTC (permalink / raw)
  To: Andreas Schwab, Libc-stable Mailing List
  Cc: H.J. Lu, Florian Weimer, GNU C Library, Arjan van de Ven,
	Paul A . Clarke

On Fri, Nov 12, 2021 at 10:53 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
>
> On Nov 11 2021, H.J. Lu wrote:
>
> > diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
> > index 57f3f28869..f763cfc7fa 100644
> > --- a/nptl/pthread_mutex_timedlock.c
> > +++ b/nptl/pthread_mutex_timedlock.c
> > @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
> >            meantime.  */
> >         if ((oldval & FUTEX_WAITERS) == 0)
> >           {
> > -           if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > -                                                     oldval | FUTEX_WAITERS,
> > -                                                     oldval)
> > -               != 0)
> > +           int val;
> > +           if ((val = atomic_compare_and_exchange_val_acq
> > +                (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > +                 oldval)) != oldval)
>
> Please move the assignment out of the condition.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> "And now for something completely different."

I would like to backport this patch to release branch 2.33 and 2.34

Any comments/suggestions or objections on this.

commit 0b82747dc48d5bf0871bdc6da8cb6eec1256355f
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Nov 11 06:31:51 2021 -0800

    Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]

    Replace boolean CAS with value CAS to avoid the extra load.

    Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]
  2022-09-11 20:12     ` [PATCH v6 4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537] Sunil Pandey
@ 2022-09-11 20:15       ` Arjan van de Ven
  2022-09-11 21:26         ` Florian Weimer
  2022-09-29  0:09       ` Noah Goldstein
  1 sibling, 1 reply; 8+ messages in thread
From: Arjan van de Ven @ 2022-09-11 20:15 UTC (permalink / raw)
  To: Sunil Pandey, Andreas Schwab, Libc-stable Mailing List
  Cc: H.J. Lu, Florian Weimer, GNU C Library, Paul A . Clarke

On 9/11/2022 1:12 PM, Sunil Pandey wrote:
> On Fri, Nov 12, 2021 at 10:53 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
>>
>> On Nov 11 2021, H.J. Lu wrote:
>>
>>> diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
>>> index 57f3f28869..f763cfc7fa 100644
>>> --- a/nptl/pthread_mutex_timedlock.c
>>> +++ b/nptl/pthread_mutex_timedlock.c
>>> @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
>>>             meantime.  */
>>>          if ((oldval & FUTEX_WAITERS) == 0)
>>>            {
>>> -           if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
>>> -                                                     oldval | FUTEX_WAITERS,
>>> -                                                     oldval)
>>> -               != 0)
>>> +           int val;
>>> +           if ((val = atomic_compare_and_exchange_val_acq
>>> +                (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
>>> +                 oldval)) != oldval)
>>
>> Please move the assignment out of the condition.
>>
>> Andreas.
>>
>> --
>> Andreas Schwab, schwab@linux-m68k.org
>> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
>> "And now for something completely different."
> 
> I would like to backport this patch to release branch 2.33 and 2.34
> 

what exactly is the stable branch policy that would suggest to backport performance improvements like this ?
(most projects are sticking to "strict bugfixes and other gross oversights" as much as possible)


^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 2/4] Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]
       [not found]   ` <87zgq9rywl.fsf@igel.home>
@ 2022-09-11 20:16     ` Sunil Pandey
  2022-09-29  0:10       ` Noah Goldstein
  0 siblings, 1 reply; 8+ messages in thread
From: Sunil Pandey @ 2022-09-11 20:16 UTC (permalink / raw)
  To: Andreas Schwab, Libc-stable Mailing List
  Cc: H.J. Lu, Florian Weimer, GNU C Library, Arjan van de Ven,
	Paul A . Clarke

On Fri, Nov 12, 2021 at 10:52 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
>
> On Nov 11 2021, H.J. Lu wrote:
>
> > diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> > index 72058c719c..762059b230 100644
> > --- a/nptl/pthread_mutex_lock.c
> > +++ b/nptl/pthread_mutex_lock.c
> > @@ -304,12 +304,12 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
> >            meantime.  */
> >         if ((oldval & FUTEX_WAITERS) == 0)
> >           {
> > -           if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > -                                                     oldval | FUTEX_WAITERS,
> > -                                                     oldval)
> > -               != 0)
> > +           int val;
> > +           if ((val = atomic_compare_and_exchange_val_acq
> > +                (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > +                 oldval)) != oldval)
>
> Please move the assignment out of the condition.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> "And now for something completely different."

I would like to backport this patch to release branch 2.33 and 2.34

Any comments/suggestions or objections on this.

commit 49302b8fdf9103b6fc0a398678668a22fa19574c
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Nov 11 06:54:01 2021 -0800

    Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]

    Replace boolean CAS with value CAS to avoid the extra load.

    Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 1/4] Add LLL_MUTEX_READ_LOCK [BZ #28537]
       [not found]           ` <924a80cd-202c-99e9-a2d9-1aeda2235b83@linux.intel.com>
@ 2022-09-11 20:19             ` Sunil Pandey
  2022-09-29  0:10               ` Noah Goldstein
  0 siblings, 1 reply; 8+ messages in thread
From: Sunil Pandey @ 2022-09-11 20:19 UTC (permalink / raw)
  To: Arjan van de Ven, Libc-stable Mailing List
  Cc: H.J. Lu, Noah Goldstein, Florian Weimer, Andreas Schwab,
	GNU C Library, Paul A . Clarke

On Wed, Nov 17, 2021 at 5:17 PM Arjan van de Ven via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On 11/17/2021 4:31 PM, H.J. Lu wrote:
> >> Yes, but the loop will be able to run `max_cnt` iterations much faster now.
> >> Just wondering if the value needs to be re-tuned. Not that is necessarily needs
> >> to be.
> > Maybe if we can find some data to show for.
> >
>
> wondering when this was last tuned.. I assume todays CPUs and CPUs from, say, 5 or 10 years ago
> have an order of magnitude different performance in this regard....
> if there wasn't a need to retune during that, maybe this value is so robust that it doesn't need
> retuning.
>
> or maybe it's time to retune this in general sometime soon after this patch goes in ;)

I would like to backport this patch to release branch 2.33 and 2.34

Any comments/suggestions or objections on this.

commit d672a98a1af106bd68deb15576710cd61363f7a6
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Tue Nov 2 18:33:07 2021 -0700

    Add LLL_MUTEX_READ_LOCK [BZ #28537]

    CAS instruction is expensive.  From the x86 CPU's point of view, getting
    a cache line for writing is more expensive than reading.  See Appendix
    A.2 Spinlock in:

    https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]
  2022-09-11 20:15       ` Arjan van de Ven
@ 2022-09-11 21:26         ` Florian Weimer
  0 siblings, 0 replies; 8+ messages in thread
From: Florian Weimer @ 2022-09-11 21:26 UTC (permalink / raw)
  To: Arjan van de Ven via Libc-alpha
  Cc: Sunil Pandey, Andreas Schwab, Libc-stable Mailing List,
	Arjan van de Ven, Paul A . Clarke

* Arjan van de Ven via Libc-alpha:

> what exactly is the stable branch policy that would suggest to
> backport performance improvements like this ?  (most projects are
> sticking to "strict bugfixes and other gross oversights" as much as
> possible)

We occasionally backport safe (and not-so-safe) performance
optimizations.  We draw the line at ABI changes, those aren't possible.
It's also very desirable that all future releases have a superset of the
backports.

But anything else depends on who is willing to do the work.

Thanks,
Florian


^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]
  2022-09-11 20:12     ` [PATCH v6 4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537] Sunil Pandey
  2022-09-11 20:15       ` Arjan van de Ven
@ 2022-09-29  0:09       ` Noah Goldstein
  1 sibling, 0 replies; 8+ messages in thread
From: Noah Goldstein @ 2022-09-29  0:09 UTC (permalink / raw)
  To: Sunil Pandey
  Cc: Andreas Schwab, Libc-stable Mailing List, Florian Weimer,
	Arjan van de Ven, GNU C Library, Paul A . Clarke

On Sun, Sep 11, 2022 at 4:13 PM Sunil Pandey via Libc-stable
<libc-stable@sourceware.org> wrote:
>
> On Fri, Nov 12, 2021 at 10:53 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
> >
> > On Nov 11 2021, H.J. Lu wrote:
> >
> > > diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
> > > index 57f3f28869..f763cfc7fa 100644
> > > --- a/nptl/pthread_mutex_timedlock.c
> > > +++ b/nptl/pthread_mutex_timedlock.c
> > > @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
> > >            meantime.  */
> > >         if ((oldval & FUTEX_WAITERS) == 0)
> > >           {
> > > -           if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > > -                                                     oldval | FUTEX_WAITERS,
> > > -                                                     oldval)
> > > -               != 0)
> > > +           int val;
> > > +           if ((val = atomic_compare_and_exchange_val_acq
> > > +                (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > > +                 oldval)) != oldval)
> >
> > Please move the assignment out of the condition.
> >
> > Andreas.
> >
> > --
> > Andreas Schwab, schwab@linux-m68k.org
> > GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> > "And now for something completely different."
>
> I would like to backport this patch to release branch 2.33 and 2.34
>
> Any comments/suggestions or objections on this.
>
> commit 0b82747dc48d5bf0871bdc6da8cb6eec1256355f
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Thu Nov 11 06:31:51 2021 -0800
>
>     Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]
>
>     Replace boolean CAS with value CAS to avoid the extra load.
>
>     Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

Fine by me.

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 1/4] Add LLL_MUTEX_READ_LOCK [BZ #28537]
  2022-09-11 20:19             ` [PATCH v6 1/4] Add LLL_MUTEX_READ_LOCK " Sunil Pandey
@ 2022-09-29  0:10               ` Noah Goldstein
  0 siblings, 0 replies; 8+ messages in thread
From: Noah Goldstein @ 2022-09-29  0:10 UTC (permalink / raw)
  To: Sunil Pandey
  Cc: Arjan van de Ven, Libc-stable Mailing List, H.J. Lu,
	Florian Weimer, Andreas Schwab, GNU C Library, Paul A . Clarke

On Sun, Sep 11, 2022 at 4:19 PM Sunil Pandey <skpgkp2@gmail.com> wrote:
>
> On Wed, Nov 17, 2021 at 5:17 PM Arjan van de Ven via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > On 11/17/2021 4:31 PM, H.J. Lu wrote:
> > >> Yes, but the loop will be able to run `max_cnt` iterations much faster now.
> > >> Just wondering if the value needs to be re-tuned. Not that is necessarily needs
> > >> to be.
> > > Maybe if we can find some data to show for.
> > >
> >
> > wondering when this was last tuned.. I assume todays CPUs and CPUs from, say, 5 or 10 years ago
> > have an order of magnitude different performance in this regard....
> > if there wasn't a need to retune during that, maybe this value is so robust that it doesn't need
> > retuning.
> >
> > or maybe it's time to retune this in general sometime soon after this patch goes in ;)
>
> I would like to backport this patch to release branch 2.33 and 2.34

Fine by  me.
>
> Any comments/suggestions or objections on this.
>
> commit d672a98a1af106bd68deb15576710cd61363f7a6
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Tue Nov 2 18:33:07 2021 -0700
>
>     Add LLL_MUTEX_READ_LOCK [BZ #28537]
>
>     CAS instruction is expensive.  From the x86 CPU's point of view, getting
>     a cache line for writing is more expensive than reading.  See Appendix
>     A.2 Spinlock in:
>
>     https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [PATCH v6 2/4] Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]
  2022-09-11 20:16     ` [PATCH v6 2/4] Avoid extra load with CAS in __pthread_mutex_lock_full " Sunil Pandey
@ 2022-09-29  0:10       ` Noah Goldstein
  0 siblings, 0 replies; 8+ messages in thread
From: Noah Goldstein @ 2022-09-29  0:10 UTC (permalink / raw)
  To: Sunil Pandey
  Cc: Andreas Schwab, Libc-stable Mailing List, Florian Weimer,
	Arjan van de Ven, GNU C Library, Paul A . Clarke

On Sun, Sep 11, 2022 at 4:17 PM Sunil Pandey via Libc-stable
<libc-stable@sourceware.org> wrote:
>
> On Fri, Nov 12, 2021 at 10:52 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
> >
> > On Nov 11 2021, H.J. Lu wrote:
> >
> > > diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> > > index 72058c719c..762059b230 100644
> > > --- a/nptl/pthread_mutex_lock.c
> > > +++ b/nptl/pthread_mutex_lock.c
> > > @@ -304,12 +304,12 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
> > >            meantime.  */
> > >         if ((oldval & FUTEX_WAITERS) == 0)
> > >           {
> > > -           if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > > -                                                     oldval | FUTEX_WAITERS,
> > > -                                                     oldval)
> > > -               != 0)
> > > +           int val;
> > > +           if ((val = atomic_compare_and_exchange_val_acq
> > > +                (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > > +                 oldval)) != oldval)
> >
> > Please move the assignment out of the condition.
> >
> > Andreas.
> >
> > --
> > Andreas Schwab, schwab@linux-m68k.org
> > GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> > "And now for something completely different."
>
> I would like to backport this patch to release branch 2.33 and 2.34
>
> Any comments/suggestions or objections on this.
>
> commit 49302b8fdf9103b6fc0a398678668a22fa19574c
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Thu Nov 11 06:54:01 2021 -0800
>
>     Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]
>
>     Replace boolean CAS with value CAS to avoid the extra load.
>
>     Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

Fine by me.

^ permalink raw reply	[flat|nested] 8+ messages in thread

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