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From: Dave Martin <Dave.Martin@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Julien Grall <julien@xen.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	zhang.lei@jp.fujitsu.com, Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	Daniel Kiss <Daniel.Kiss@arm.com>
Subject: Re: [PATCH v3 0/8] arm64/sve: First steps towards optimizing syscalls
Date: Mon, 20 Jul 2020 11:44:40 +0100	[thread overview]
Message-ID: <20200720104438.GH30452@arm.com> (raw)
In-Reply-To: <20200715171116.GK5431@sirena.org.uk>

On Wed, Jul 15, 2020 at 06:11:16PM +0100, Mark Brown wrote:
> On Wed, Jul 15, 2020 at 05:49:34PM +0100, Dave Martin wrote:
> 
> > I wonder whether we ought to accompany this with a crude mechanism to
> > choose dynamically between setting TIF_SVE_NEEDS_FLUSH and keeping the
> > old behaviour.
> 
> > My concern with doing this unconditionally has been that we can end up
> > with TIF_SVE permanently stuck on, which increases the per-task overhead.
> > This is not a worry if the user task really does use SVE once per
> > context switch, but not so good if, say, the libc startup probes for
> > SVE to initialise some internal logic but the task otherwise doesn't
> > use it.  (This is just a worry: I haven't looked for evidence to support
> > it.)
> 
> Yes, it's a concern.  My thought was to follow this up with something
> which copies what some of the other architectures are doing for FP
> registers and go back to the existing behaviour after a certain number
> of syscalls.  That has a bunch of room for debate and bikeshedding about
> what exactly an appropriate number might be of course.

Ack, that sounds like a fair approach.

Of course, we could try to implement some kind of clever backoff but
it's probably not worth it for now.

Cheers
---Dave

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  reply	other threads:[~2020-07-20 10:46 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-29 13:35 [PATCH v3 0/8] arm64/sve: First steps towards optimizing syscalls Mark Brown
2020-06-29 13:35 ` [PATCH v3 1/8] arm64/fpsimd: Update documentation of do_sve_acc Mark Brown
2020-06-29 13:35 ` [PATCH v3 2/8] arm64/signal: Update the comment in preserve_sve_context Mark Brown
2020-06-29 13:35 ` [PATCH v3 3/8] arm64/fpsimdmacros: Allow the macro "for" to be used in more cases Mark Brown
2020-06-29 13:35 ` [PATCH v3 4/8] arm64/fpsimdmacros: Introduce a macro to update ZCR_EL1.LEN Mark Brown
2020-06-29 13:35 ` [PATCH v3 5/8] arm64/sve: Implement a helper to flush SVE registers Mark Brown
2020-07-15 16:52   ` Dave Martin
2020-06-29 13:35 ` [PATCH v3 6/8] arm64/sve: Implement a helper to load SVE registers from FPSIMD state Mark Brown
2020-07-15 16:52   ` Dave Martin
2020-06-29 13:35 ` [PATCH v3 7/8] arm64/sve: Don't disable SVE on syscalls return Mark Brown
2020-07-15 16:52   ` Dave Martin
2020-08-21 21:54     ` Mark Brown
2020-06-29 13:35 ` [PATCH v3 8/8] arm64/sve: Rework SVE trap access to use TIF_SVE_NEEDS_FLUSH Mark Brown
2020-07-15 16:52   ` Dave Martin
2020-07-15 16:49 ` [PATCH v3 0/8] arm64/sve: First steps towards optimizing syscalls Dave Martin
2020-07-15 17:11   ` Mark Brown
2020-07-20 10:44     ` Dave Martin [this message]
2020-07-21  2:43       ` zhang.lei
2020-07-21 22:34         ` Mark Brown

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