Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible

[Davidlohr Bueso]

On Fri, 20 Jul 2018, Andrew Morton wrote:

> On Fri, 20 Jul 2018 10:29:54 -0700 Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
>
> > Hi,
> >
> > Both patches replace saving+restoring interrupts when taking the
> > ep->lock (now the waitqueue lock), with just disabling local irqs.
> > This shows immediate performance benefits in patch 1 for an epoll
> > workload running on Xen.
>
> I'm surprised.  Is spin_lock_irqsave() significantly more expensive
> than spin_lock_irq()?  Relative to all the other stuff those functions
> are doing?  If so, how come?  Some architectural thing makes
> local_irq_save() much more costly than local_irq_disable()?

For example, if you compare x86 native_restore_fl() to xen_restore_fl(),
the cost of Xen is much higher.

And at least considering ep_scan_ready_list(), the lock is taken/released
twice, to deal with the ovflist when the ep->wq.lock is not held. To the
point that it yields measurable results (see patch 1) across incremental
thread counts.

> > The main concern we need to have with this
> > sort of changes in epoll is the ep_poll_callback() which is passed
> > to the wait queue wakeup and is done very often under irq context,
> > this patch does not touch this call.
>
> Yeah, these changes are scary.  For the code as it stands now, and for
> the code as it evolves.

Yes which is why I've been throwing lots of epoll workloads at it.

> I'd have more confidence if we had some warning mechanism if we run
> spin_lock_irq() when IRQs are disabled, which is probably-a-bug.  But
> afaict we don't have that.  Probably for good reasons - I wonder what
> they are?
>
> > Patches have been tested pretty heavily with the customer workload,
> > microbenchmarks, ltp testcases and two high level workloads that
> > use epoll under the hood: nginx and libevent benchmarks.
> >
> > Details are in the individual patches.
> >
> > Applies on top of mmotd.
>
> Please convince me about the performance benefits?

As for number I only have patch 1.

Thanks,
Davidlohr