linux-kernel - Re: [lkp] [ipc/sem.c] 5864a2fd30: aim9.shared_memory.ops_per

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Message-Id: <20161019172102.d04a40c4e2f9d8054aa7ec78@linux-foundation.org>
Date:   Wed, 19 Oct 2016 17:21:02 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
        1vier1@....de, torvalds@...ux-foundation.org,
        xiaolong.ye@...el.com, felixh@...ormatik.uni-bremen.de
Subject: Re: [lkp] [ipc/sem.c]  5864a2fd30:  aim9.shared_memory.ops_per_sec
 -13.0%

On Wed, 19 Oct 2016 06:38:14 +0200 Manfred Spraul <manfred@...orfullife.com> wrote:

> Hi,
> 
> as discussed before:
> The root cause for the performance regression is the smp_mb() that was
> added into the fast path.
> 
> I see two options:
> 1) switch to full spin_lock()/spin_unlock() for the rare codepath,
>   then the fast path doesn't need the smp_mb() anymore.
> 
> 2) confirm that no arch needs the smp_mb(), then remove it.
>   - powerpc is ok after commit
>      6262db7c088b ("powerpc/spinlock: Fix spin_unlock_wait()")
>   - arm is ok after commit
>      d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers")
>   - for x86 is ok after commit
>      2c6100227116 ("locking/qspinlock: Fix spin_unlock_wait() some more")
>   - for the remaining SMP architectures, I don't have a status.
> 
> I would prefer the approach 1:
> The memory ordering provided by spin_lock()/spin_unlock() is clear.
> 
> Thus:
> Attached are patches for approach 1:
> 
> - Patch 1 replaces spin_unlock_wait() with spin_lock()/spin_unlock() and
>   removes all memory barriers that are then unnecessary.
> 
> - Patch 2 adds the hysteresis code: This makes the rare codepath
>   extremely rare.
>   It also corrects some wrong comments, e.g. regarding switching
>   from global lock to per-sem lock (we "must' switch, not we "can"
>   switch as written right now).
> 
> The patches passed stress-testing.
> 
> What do you think?

Are you able to confirm that the performance issues are fixed?

> My initial idea was to aim for 4.10, then we have more time to decide.

I suppose I can slip these into -next and see what the effect is upon
the Intel test results.  But a) I don't know if they test linux-next(?)
and b) I don't know where the test results are published, assuming they
are published(?).