From: "Paul A. Clarke" <pc@us.ibm.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: "H.J. Lu" <hjl.tools@gmail.com>,
libc-alpha@sourceware.org,
Arjan van de Ven <arjan@linux.intel.com>,
Andreas Schwab <schwab@linux-m68k.org>
Subject: Re: [PATCH v4 0/3] Optimize CAS [BZ #28537]
Date: Wed, 10 Nov 2021 10:04:43 -0600 [thread overview]
Message-ID: <20211110160443.GE4930@li-24c3614c-2adc-11b2-a85c-85f334518bdb.ibm.com> (raw)
In-Reply-To: <87o86s2ekj.fsf@oldenburg.str.redhat.com>
On Wed, Nov 10, 2021 at 04:52:28PM +0100, Florian Weimer wrote:
> * Paul A. Clarke:
>
> > On Tue, Nov 09, 2021 at 04:16:11PM -0800, H.J. Lu via Libc-alpha wrote:
> >> 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
> >>
> >> The full compare and swap will grab the cache line exclusive and cause
> >> excessive cache line bouncing.
> >>
> >> Optimize CAS in low level locks and pthread_mutex_lock.c:
> >>
> >> 1. Do an atomic load and skip CAS if compare may fail to reduce cache
> >> line bouncing on contended locks.
> >> 2. Replace atomic_compare_and_exchange_bool_acq with
> >> atomic_compare_and_exchange_val_acq to avoid the extra load.
> >> 3. Drop __glibc_unlikely in __lll_trylock and lll_cond_trylock since we
> >> don't know if it's actually rare; in the contended case it is clearly not
> >> rare.
> >
> > I see build errors:
> >
> > In file included from pthread_mutex_cond_lock.c:23:
> > ../nptl/pthread_mutex_lock.c: In function ‘__pthread_mutex_cond_lock_full’:
> > ../nptl/pthread_mutex_lock.c:442:6: error: a label can only be part of a statement and a declaration is not a statement
> > int private = (robust
> > ^~~
>
> The patch has:
>
> + locked_mutex:
> /* The mutex is locked. The kernel will now take care of
> everything. */
> int private = (robust
>
> This is only supported in recent C versions, I think the workaround is
> to add an empty statement with a semicolon, like this:
>
> + locked_mutex:;
> /* The mutex is locked. The kernel will now take care of
> everything. */
> int private = (robust
This change resolved the issue.
PC
next prev parent reply other threads:[~2021-11-10 16:04 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-10 0:16 H.J. Lu
2021-11-10 0:16 ` [PATCH v4 1/3] Reduce CAS in low level locks " H.J. Lu
2021-11-10 1:56 ` Noah Goldstein
2021-11-10 0:16 ` [PATCH v4 2/3] Reduce CAS in __pthread_mutex_lock_full " H.J. Lu
2021-11-10 0:16 ` [PATCH v4 3/3] Optimize " H.J. Lu
2021-11-10 14:26 ` [PATCH v4 0/3] Optimize CAS " Paul E Murphy
2021-11-10 20:07 ` Paul A. Clarke
2021-11-10 21:33 ` H.J. Lu
2021-11-11 0:30 ` Paul A. Clarke
2021-11-10 23:34 ` H.J. Lu
2021-11-10 15:35 ` Paul A. Clarke
2021-11-10 15:42 ` H.J. Lu
2021-11-10 15:50 ` Paul A. Clarke
2021-11-10 15:52 ` H.J. Lu
2021-11-10 15:52 ` Florian Weimer
2021-11-10 16:03 ` H.J. Lu
2021-11-10 16:04 ` Paul A. Clarke [this message]
2021-11-10 16:14 ` Andreas Schwab
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