linux-kernel - Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll

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Message-ID: <725cdce88418c2ec62ef6014d388dbeb@suse.de>
Date:   Tue, 08 Jan 2019 07:16:10 -0800
From:   Davidlohr Bueso <dbueso@...e.de>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Jason Baron <jbaron@...mai.com>, Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is
 always called with irqs off

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