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

[Tejun Heo <tj@xxxxxxxxxx>]

Hello, Dave.

On Thu, Oct 08, 2015 at 12:02:18PM +1100, Dave Chinner wrote:
> > 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....

Dude, it isn't trivially solvable.  You either can't do it or have to
pay the overhead during local access to get around it.

> > 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...

Just chipping in purely from percpu side.  If what Waiman suggested is
something useable, caching the result inside percpu_counter would be a
better interface.  If not, no idea.

Thanks.

-- 
tejun