Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH 2/2] locking/rtmutex: optimize rt_mutex_cmpxchgs | Date | Sun, 26 Apr 2020 18:31:55 +0200 |
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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
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