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

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Date:   Thu, 6 Sep 2018 13:55:17 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, jbaron@...mai.com,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible

On Thu, 06 Sep 2018, Peter Zijlstra wrote:

>On Fri, Jul 20, 2018 at 01:05:59PM -0700, Davidlohr Bueso wrote:
>> On Fri, 20 Jul 2018, Andrew Morton wrote:
>
>> >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.
>
>Xen is a moot argument. IIRC the point is that POPF (as used by
>*irqrestore()) is a very expensive operation because it changes all
>flags and thus has very 'difficult' instruction dependencies, killing
>the front end reorder and generating a giant bubble in the pipeline.
>
>Similarly, I suppose PUSHF is an expensive instruction because it needs
>all the flags 'stable' and thus needs to wait for a fair number of prior
>instructions to retire before it can get on with it.
>
>Combined the whole PUSHF + POPF is _far_ more expensive than STI + CLI,
>because the latter only has dependencies on instructions that muck about
>with IF -- not that many.

ack.

In fact it turns out that my Xen numbers for this patch were actually
native (popf), and not the xen_restore_fl() as it was using hvm and
not paravirt.

Thanks,
Davidlohr