From: Roman Penyaev <rpenyaev@suse.de>
To: Davidlohr Bueso <dbueso@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 17:07:23 +0100 [thread overview]
Message-ID: <2cec506322600c3d2b59f2ca9bcd59dc@suse.de> (raw)
In-Reply-To: <725cdce88418c2ec62ef6014d388dbeb@suse.de>
On 2019-01-08 16:16, Davidlohr Bueso wrote:
> 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.
Then I will send another patch which changes read_lock() on
read_lock_irqsave(),
since simple revert of the original patch won't work.
Thanks.
--
Roman
prev parent reply other threads:[~2019-01-08 16:07 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
2019-01-08 16:07 ` Roman Penyaev [this message]
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