linux-kernel - Re: [RFC][PATCH] x86: Lazy disabling of interrupts

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Date:	Wed, 09 Oct 2013 15:25:23 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <williams@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kleen\, Andi" <andi.kleen@...el.com>
Subject: Re: [RFC][PATCH] x86: Lazy disabling of interrupts

"H. Peter Anvin" <hpa@...ux.intel.com> writes:

>> Summary
>> -------
>> 
>> Although the extreme case shows a nice improvement, I'm skeptical if it
>> is worth doing for real world applications.
>
> You did the experiment, and credit to you for not going "I did the work,
> now include it" but rather for publishing the results so we can learn
> from them.
>
> It *does* make me wonder if we can leverage RTM for a significant subset
> of these (as an interrupt will abort a transaction); that should be
> substantially cheaper and less complex.

I miss the original context and can't find the original patchkit, but:

- If the goal is to lower interrupt latency then RTM would still
need to use a fallback, so the worst case would be the fallback, thus
not be better.

- If the goal is to make CLI/STI faster:
I'm not sure RTM is any faster than a PUSHF/CLI/POPF pair. It may
well be slightly slower in fact (guessing here, haven't benchmarked)

- Also when you abort you would need to reexecute of course.

- My TSX patchkit actually elides CLI/STI inside transactions
(no need to do them, as any interrupt would abort anyways)
but the main motivation was to avoid extra aborts.

- That said, I think a software CLI/STI is somewhat useful for profiling,
as it can allow to measure how long interrupts are delayed
by CLI/STI.  I heard use cases of this, but I'm not 
sure how common it really is

[I presume a slightly modified RT kernel could also give the same
profiling results]

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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