Re: [PATCH] percpu_counter: return precise count from __percpu_counter_compare()

[Waiman Long]

On 10/07/2015 09:02 PM, Dave Chinner wrote:
> On Wed, Oct 07, 2015 at 04:20:10PM -0700, Tejun Heo wrote:
> > Hello, Dave.
> >
> > On Thu, Oct 08, 2015 at 10:04:42AM +1100, Dave Chinner wrote:
> > ...
> > > As it is, the update race you pointed out is easy to solve with
> > > __this_cpu_cmpxchg rather than _this_cpu_sub (similar to mod_state()
> > > in the MM percpu counter stats code, perhaps).
> > percpu cmpxchg is no different from sub or any other operations
> > regarding cross-CPU synchronization.  They're safe iff the operations
> > are on the local CPU.  They have to be made atomics if they need to be
> > manipulated from remote CPUs.
> Again, another trivially solvable problem, but still irrelevant
> because we don't have the data that tells us whether changing the
> counter behaviour solves the problem....
>
> > That said, while we can't manipulate the percpu counters directly, we
> > can add a separate global counter to cache sum result from the
> > previous run which gets automatically invalidated when any percpu
> > counter overflows.
> >
> > That should give better and in case of
> > back-to-back invocations pretty good precision compared to just
> > returning the global overflow counter.  Interface-wise, that'd be a
> > lot easier to deal with although I have no idea whether it'd fit this
> > particular use case or whether this use case even exists.
> No, it doesn't help - it's effectively what Waiman's original patch
> did by returning the count from the initial comparison and using
> that for ENOSPC detection instead of doing a second comparison...
>
> FWIW, XFS has done an expensive per-cpu counter sum in this ENOSPC
> situation since 2006, but in 2007 ENOSPC was wrapped in a mutex to
> prevent spinlock contention on the aggregated global counter:
>
> commit 20b642858b6bb413976ff13ae6a35cc596967bab
> Author: David Chinner<dgc@xxxxxxx>
> Date:   Sat Feb 10 18:35:09 2007 +1100
>
>      [XFS] Reduction global superblock lock contention near ENOSPC.
>
>      The existing per-cpu superblock counter code uses the global superblock
>      spin lock when we approach ENOSPC for global synchronisation. On larger
>      machines than this code was originally tested on this can still get
>      catastrophic spinlock contention due increasing rebalance frequency near
>      ENOSPC.
>
>      By introducing a sleeping lock that is used to serialise balances and
>      modifications near ENOSPC we prevent contention from needlessly from
>      wasting the CPU time of potentially hundreds of CPUs.
>
>      To reduce the number of balances occuring, we separate the need rebalance
>      case from the slow allocate case. Now, a counter running dry will trigger
>      a rebalance during which counters are disabled. Any thread that sees a
>      disabled counter enters a different path where it waits on the new mutex.
>      When it gets the new mutex, it checks if the counter is disabled. If the
>      counter is disabled, then we _know_ that we have to use the global counter
>      and lock and it is safe to do so immediately. Otherwise, we drop the mutex
>      and go back to trying the per-cpu counters which we know were re-enabled.
>
>      SGI-PV: 952227
>      SGI-Modid: xfs-linux-melb:xfs-kern:27612a
>
>      Signed-off-by: David Chinner<dgc@xxxxxxx>
>      Signed-off-by: Lachlan McIlroy<lachlan@xxxxxxx>
>      Signed-off-by: Tim Shimmin<tes@xxxxxxx>
>
> This is effectively the same symptoms that what we are seeing with
> the new "lockless" generic percpu counteri algorithm, which is why
> I'm trying to find out if it an issue with the counter
> implementation before I do anything else...
>
> FWIW, the first comparison doesn't need to be that precise as it
> just changes the batch passed to percpu_counter_add() to get the
> value folded back into the global counter immediately near ENOSPC.
> This is done so percpu_counter_read() becomes more accurate as
> ENOSPC is approached as that is used for monitoring and reporting
> (e.g. via vfsstat). If we want to avoid a counter sum, then this
> is the comparison we will need to modify in XFS.

That is what I have advocated in the in the inlined patch that I sent 
you in a previous mail. That patch modified the first comparison, but 
leave the 2nd comparison intact. We will still see bad performance near 
ENOSPC, but it will be better than before.

> However, the second comparison needs to be precise as that's the one
> that does the ENOSPC detection. That sum needs to be done after the
> counter add that "uses" the space and so there is no avoiding having
> an expensive counter sum as we near ENOSPC....
>
> Cheers,
>
> Dave.

Cheers,
Longman
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