Re: [lkp] [ipc/sem.c] 5864a2fd30: aim9.shared_memory.ops_per_sec -13.0%

[Manfred Spraul]

On 10/20/2016 02:21 AM, Andrew Morton wrote:
> On Wed, 19 Oct 2016 06:38:14 +0200 Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> 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?
I don't know why we are now at -13%.
The previous -9% were resolved by the patches:

http://marc.info/?l=linux-kernel&m=147608082017551&w=2
> > 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(?).

--

    Manfred