lkml.org 
[lkml]   [2020]   [Apr]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH 2/2] locking/rtmutex: optimize rt_mutex_cmpxchgs
Date
Alex Shi <alex.shi@linux.alibaba.com> writes:

> Checking l->owner first to skip time cost cmpxchgs.

I don't see what that buys.

It actually adds an extra conditional in the non-contended case, which
is the case we are optimizing for.

In the contended case, i.e. when l->owner != c the cmpxchg cost is
completely irrelevant compared to the slowpath costs.

> #ifndef CONFIG_DEBUG_RT_MUTEXES
> -# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
> -# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_acquire(l, c, n) \
> + (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_release(l, c, n) \
> + (l->owner == c && cmpxchg_release(&l->owner, c, n) == c)

This kind of micro-optimizing is more than dubious especially w/o
numbers backing up the benefit.

Thanks,

tglx

\
 
 \ /
  Last update: 2020-04-26 18:32    [W:0.085 / U:1.184 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site