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

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Date:   Fri, 20 Jul 2018 17:22:54 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     jbaron@...mai.com, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible

On Fri, 20 Jul 2018, Andrew Morton wrote:

>Did you try measuring it on bare hardware?

I did and wasn't expecting much difference.

For a 2-socket 40-core (ht) IvyBridge on a few workloads, unfortunately
I don't have a xen environment and the results for Xen I do have (which numbers
are in patch 1) I don't have the actual workload, so cannot compare them directly.

1) Different configurations were used for a epoll_wait (pipes io) microbench
(http://linux-scalability.org/epoll/epoll-test.c) and shows around a 7-10%
improvement in overall total number of times the epoll_wait() loops when using
both regular and nested epolls, so very raw numbers, but measurable nonetheless.

# threads	vanilla		dirty
     1		1677717		1805587
     2		1660510		1854064
     4		1610184		1805484
     8		1577696		1751222
     16		1568837		1725299
     32		1291532		1378463
     64		 752584		 787368

Note that stddev is pretty small.

2) Another pipe test, which shows no real measurable improvement.
(http://www.xmailserver.org/linux-patches/pipetest.c)

>> >
>> >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?
>
>Well ignored ;)
>
>We could open-code it locally.  Add a couple of
>WARN_ON_ONCE(irqs_disabled())?  That might need re-benchmarking with
>Xen but surely just reading the thing isn't too expensive?

I agree, I'll see what I can come up with and also ask the customer to test
in his setup. Bare metal would also need some new numbers I guess.

Thanks,
Davidlohr