From: Davidlohr Bueso <dbueso@suse.de>
To: Roman Penyaev <rpenyaev@suse.de>
Cc: Jason Baron <jbaron@akamai.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off
Date: Tue, 08 Jan 2019 07:16:10 -0800 [thread overview]
Message-ID: <725cdce88418c2ec62ef6014d388dbeb@suse.de> (raw)
In-Reply-To: <f582f40b9697dced81d16b3b35a4b071@suse.de>
On 2019-01-08 04:42, Roman Penyaev wrote:
> What we can do:
>
> a) disable irqs if we are not in interrupt.
> b) revert the patch completely.
>
> David, is it really crucial in terms of performance to avoid double
> local_irq_save() on Xen on this ep_poll_callback() hot path?
Note that such optimizations are also relevant for baremetal, ie: x86
PUSHF + POPF can be pretty expensive because of insn dependencies.
>
> For example why not to do the following:
>
> if (!in_interrupt())
> local_irq_save(flags);
> read_lock(ep->lock);
>
> with huge comment explaining performance number.
>
> Or just give up and simply revert the original patch completely
> and always call read_lock_irqsave().
Yeah so the reason why I had done the other epoll lock irq optimizations
was because they were painfully obvious. ep_poll_callback(), however is
a different beast, as you've encountered. I vote for not shooting
ourselves in the foot and just dropping this patch -- most large
performance benefits will come from microbenches anyway.
Thanks,
Davidlohr
next prev parent reply other threads:[~2019-01-08 15:16 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-08 10:01 [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off Roman Penyaev
2019-01-08 12:42 ` Roman Penyaev
2019-01-08 15:16 ` Davidlohr Bueso [this message]
2019-01-08 16:07 ` Roman Penyaev
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