linux-kernel - Re: x86 memcpy performance

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Date:	Mon, 15 Aug 2011 17:29:54 +0200 (CEST)
From:	"Borislav Petkov" <bp@...en8.de>
To:	"Andy Lutomirski" <luto@....EDU>
Cc:	"Borislav Petkov" <bp@...en8.de>,
	"melwyn lobo" <linux.melwyn@...il.com>,
	"Denys Vlasenko" <vda.linux@...glemail.com>,
	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>, borislav.petkov@....com
Subject: Re: x86 memcpy performance

On Mon, 15 August, 2011 4:59 pm, Andy Lutomirski wrote:
>>> So what is the reason we cannot use sse_memcpy in interrupt context.
>>> (fpu registers not saved ? )
>>
>> Because, AFAICT, when we handle an #NM exception while running
>> sse_memcpy in an IRQ handler, we might need to allocate FPU save state
>> area, which in turn, can sleep. Then, we might get another IRQ while
>> sleeping and we should be deadlocked.
>>
>> But let me stress on the "AFAICT" above, someone who actually knows the
>> FPU code should correct me if I'm missing something.
>
> I don't think you ever get #NM as a result of kernel_fpu_begin, but you
> can certainly have problems when kernel_fpu_begin nests by accident.
> There's irq_fpu_usable() for this.
>
> (irq_fpu_usable() reads cr0 sometimes and I suspect it can be slow.)

Oh I didn't know about irq_fpu_usable(), thanks.

But still, irq_fpu_usable() still checks !in_interrupt() which means
that we don't want to run SSE instructions in IRQ context. OTOH, we
still are fine when running with CR0.TS. So what happens when we get an
#NM as a result of executing an FPU instruction in an IRQ handler? We
will have to do init_fpu() on the current task if the last hasn't used
math yet and do the slab allocation of the FPU context area (I'm looking
at math_state_restore, btw).

Thanks.

-- 
Regards/Gruss,
Boris.

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