FUCK FUCK FUCK ________________________________ From: Libc-stable on behalf of Noah Goldstein via Libc-stable Sent: Wednesday, September 28, 2022 7:12 PM To: Sunil Pandey Cc: Florian Weimer ; GNU C Library ; Wangyang Guo ; Libc-stable Mailing List Subject: Re: [PATCH v5] nptl: Add backoff mechanism to spinlock loop On Sun, Sep 11, 2022 at 4:30 PM Sunil Pandey via Libc-stable wrote: > > On Thu, May 5, 2022 at 8:07 PM H.J. Lu via Libc-alpha > wrote: > > > > On Thu, May 5, 2022 at 6:50 PM Wangyang Guo wrote: > > > > > > When mutiple threads waiting for lock at the same time, once lock owner > > > releases the lock, waiters will see lock available and all try to lock, > > > which may cause an expensive CAS storm. > > > > > > Binary exponential backoff with random jitter is introduced. As try-lock > > > attempt increases, there is more likely that a larger number threads > > > compete for adaptive mutex lock, so increase wait time in exponential. > > > A random jitter is also added to avoid synchronous try-lock from other > > > threads. > > > > > > v2: Remove read-check before try-lock for performance. > > > > > > v3: > > > 1. Restore read-check since it works well in some platform. > > > 2. Make backoff arch dependent, and enable it for x86_64. > > > 3. Limit max backoff to reduce latency in large critical section. > > > > > > v4: Fix strict-prototypes error in sysdeps/nptl/pthread_mutex_backoff.h > > > > > > v5: Commit log updated for regression in large critical section. > > > > > > Result of pthread-mutex-locks bench > > > > > > Test Platform: Xeon 8280L (2 socket, 112 CPUs in total) > > > First Row: thread number > > > First Col: critical section length > > > Values: backoff vs upstream, time based, low is better > > > > > > non-critical-length: 1 > > > 1 2 4 8 16 32 64 112 140 > > > 0 0.99 0.58 0.52 0.49 0.43 0.44 0.46 0.52 0.54 > > > 1 0.98 0.43 0.56 0.50 0.44 0.45 0.50 0.56 0.57 > > > 2 0.99 0.41 0.57 0.51 0.45 0.47 0.48 0.60 0.61 > > > 4 0.99 0.45 0.59 0.53 0.48 0.49 0.52 0.64 0.65 > > > 8 1.00 0.66 0.71 0.63 0.56 0.59 0.66 0.72 0.71 > > > 16 0.97 0.78 0.91 0.73 0.67 0.70 0.79 0.80 0.80 > > > 32 0.95 1.17 0.98 0.87 0.82 0.86 0.89 0.90 0.90 > > > 64 0.96 0.95 1.01 1.01 0.98 1.00 1.03 0.99 0.99 > > > 128 0.99 1.01 1.01 1.17 1.08 1.12 1.02 0.97 1.02 > > > > > > non-critical-length: 32 > > > 1 2 4 8 16 32 64 112 140 > > > 0 1.03 0.97 0.75 0.65 0.58 0.58 0.56 0.70 0.70 > > > 1 0.94 0.95 0.76 0.65 0.58 0.58 0.61 0.71 0.72 > > > 2 0.97 0.96 0.77 0.66 0.58 0.59 0.62 0.74 0.74 > > > 4 0.99 0.96 0.78 0.66 0.60 0.61 0.66 0.76 0.77 > > > 8 0.99 0.99 0.84 0.70 0.64 0.66 0.71 0.80 0.80 > > > 16 0.98 0.97 0.95 0.76 0.70 0.73 0.81 0.85 0.84 > > > 32 1.04 1.12 1.04 0.89 0.82 0.86 0.93 0.91 0.91 > > > 64 0.99 1.15 1.07 1.00 0.99 1.01 1.05 0.99 0.99 > > > 128 1.00 1.21 1.20 1.22 1.25 1.31 1.12 1.10 0.99 > > > > > > non-critical-length: 128 > > > 1 2 4 8 16 32 64 112 140 > > > 0 1.02 1.00 0.99 0.67 0.61 0.61 0.61 0.74 0.73 > > > 1 0.95 0.99 1.00 0.68 0.61 0.60 0.60 0.74 0.74 > > > 2 1.00 1.04 1.00 0.68 0.59 0.61 0.65 0.76 0.76 > > > 4 1.00 0.96 0.98 0.70 0.63 0.63 0.67 0.78 0.77 > > > 8 1.01 1.02 0.89 0.73 0.65 0.67 0.71 0.81 0.80 > > > 16 0.99 0.96 0.96 0.79 0.71 0.73 0.80 0.84 0.84 > > > 32 0.99 0.95 1.05 0.89 0.84 0.85 0.94 0.92 0.91 > > > 64 1.00 0.99 1.16 1.04 1.00 1.02 1.06 0.99 0.99 > > > 128 1.00 1.06 0.98 1.14 1.39 1.26 1.08 1.02 0.98 > > > > > > There is regression in large critical section. But adaptive mutex is > > > aimed for "quick" locks. Small critical section is more common when > > > users choose to use adaptive pthread_mutex. > > > > > > Signed-off-by: Wangyang Guo > > > --- > > > nptl/pthread_mutex_lock.c | 16 +++++++-- > > > sysdeps/nptl/pthreadP.h | 1 + > > > sysdeps/nptl/pthread_mutex_backoff.h | 35 ++++++++++++++++++ > > > sysdeps/x86_64/nptl/pthread_mutex_backoff.h | 39 +++++++++++++++++++++ > > > 4 files changed, 89 insertions(+), 2 deletions(-) > > > create mode 100644 sysdeps/nptl/pthread_mutex_backoff.h > > > create mode 100644 sysdeps/x86_64/nptl/pthread_mutex_backoff.h > > > > > > diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c > > > index d2e652d151..6e767a8724 100644 > > > --- a/nptl/pthread_mutex_lock.c > > > +++ b/nptl/pthread_mutex_lock.c > > > @@ -138,14 +138,26 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex) > > > int cnt = 0; > > > int max_cnt = MIN (max_adaptive_count (), > > > mutex->__data.__spins * 2 + 10); > > > + int spin_count, exp_backoff = 1; > > > + unsigned int jitter = get_jitter (); > > > do > > > { > > > - if (cnt++ >= max_cnt) > > > + /* In each loop, spin count is exponential backoff plus > > > + random jitter, random range is [0, exp_backoff-1]. */ > > > + spin_count = exp_backoff + (jitter & (exp_backoff - 1)); > > > + cnt += spin_count; > > > + if (cnt >= max_cnt) > > > { > > > + /* If cnt exceeds max spin count, just go to wait > > > + queue. */ > > > LLL_MUTEX_LOCK (mutex); > > > break; > > > } > > > - atomic_spin_nop (); > > > + do > > > + atomic_spin_nop (); > > > + while (--spin_count > 0); > > > + /* Prepare for next loop. */ > > > + exp_backoff = get_next_backoff (exp_backoff); > > > } > > > while (LLL_MUTEX_READ_LOCK (mutex) != 0 > > > || LLL_MUTEX_TRYLOCK (mutex) != 0); > > > diff --git a/sysdeps/nptl/pthreadP.h b/sysdeps/nptl/pthreadP.h > > > index 601db4ff2b..39af275c25 100644 > > > --- a/sysdeps/nptl/pthreadP.h > > > +++ b/sysdeps/nptl/pthreadP.h > > > @@ -33,6 +33,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include "pthread_mutex_conf.h" > > > > > > > > > diff --git a/sysdeps/nptl/pthread_mutex_backoff.h b/sysdeps/nptl/pthread_mutex_backoff.h > > > new file mode 100644 > > > index 0000000000..5b26c22ac7 > > > --- /dev/null > > > +++ b/sysdeps/nptl/pthread_mutex_backoff.h > > > @@ -0,0 +1,35 @@ > > > +/* Pthread mutex backoff configuration. > > > + Copyright (C) 2022 Free Software Foundation, Inc. > > > + This file is part of the GNU C Library. > > > + > > > + The GNU C Library is free software; you can redistribute it and/or > > > + modify it under the terms of the GNU Lesser General Public > > > + License as published by the Free Software Foundation; either > > > + version 2.1 of the License, or (at your option) any later version. > > > + > > > + The GNU C Library is distributed in the hope that it will be useful, > > > + but WITHOUT ANY WARRANTY; without even the implied warranty of > > > + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU > > > + Lesser General Public License for more details. > > > + > > > + You should have received a copy of the GNU Lesser General Public > > > + License along with the GNU C Library; if not, see > > > + . */ > > > +#ifndef _PTHREAD_MUTEX_BACKOFF_H > > > +#define _PTHREAD_MUTEX_BACKOFF_H 1 > > > + > > > +static inline unsigned int > > > +get_jitter (void) > > > +{ > > > + /* Arch dependent random jitter, return 0 disables random. */ > > > + return 0; > > > +} > > > + > > > +static inline int > > > +get_next_backoff (int backoff) > > > +{ > > > + /* Next backoff, return 1 disables mutex backoff. */ > > > + return 1; > > > +} > > > + > > > +#endif > > > diff --git a/sysdeps/x86_64/nptl/pthread_mutex_backoff.h b/sysdeps/x86_64/nptl/pthread_mutex_backoff.h > > > new file mode 100644 > > > index 0000000000..ec74c3d9db > > > --- /dev/null > > > +++ b/sysdeps/x86_64/nptl/pthread_mutex_backoff.h > > > @@ -0,0 +1,39 @@ > > > +/* Pthread mutex backoff configuration. > > > + Copyright (C) 2022 Free Software Foundation, Inc. > > > + This file is part of the GNU C Library. > > > + > > > + The GNU C Library is free software; you can redistribute it and/or > > > + modify it under the terms of the GNU Lesser General Public > > > + License as published by the Free Software Foundation; either > > > + version 2.1 of the License, or (at your option) any later version. > > > + > > > + The GNU C Library is distributed in the hope that it will be useful, > > > + but WITHOUT ANY WARRANTY; without even the implied warranty of > > > + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU > > > + Lesser General Public License for more details. > > > + > > > + You should have received a copy of the GNU Lesser General Public > > > + License along with the GNU C Library; if not, see > > > + . */ > > > +#ifndef _PTHREAD_MUTEX_BACKOFF_H > > > +#define _PTHREAD_MUTEX_BACKOFF_H 1 > > > + > > > +#include > > > + > > > +static inline unsigned int > > > +get_jitter (void) > > > +{ > > > + return get_fast_jitter (); > > > +} > > > + > > > +#define MAX_BACKOFF 16 > > > + > > > +static inline int > > > +get_next_backoff (int backoff) > > > +{ > > > + /* Binary expontial backoff. Limiting max backoff > > > + can reduce latency in large critical section. */ > > > + return (backoff < MAX_BACKOFF) ? backoff << 1 : backoff; > > > +} > > > + > > > +#endif > > > -- > > > 2.35.1 > > > > > > > LGTM. > > > > Reviewed-by: H.J. Lu > > > > Please wait until next Monday for more comments. > > > > Thanks. > > > > -- > > H.J. > > I would like to backport this patch to release branch 2.33, 2.34 and 2.35 Fine by me. > > Any comments/suggestions or objections on this. > > commit 8162147872491bb5b48e91543b19c49a29ae6b6d > Author: Wangyang Guo > Date: Fri May 6 01:50:10 2022 +0000 > > nptl: Add backoff mechanism to spinlock loop > > When mutiple threads waiting for lock at the same time, once lock owner > releases the lock, waiters will see lock available and all try to lock, > which may cause an expensive CAS storm.