From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Dave Martin <dave.martin@arm.com>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] arm64/fpsimd: Suppress SVE access traps when loading FPSIMD state
Date: Wed, 6 Mar 2024 18:54:51 +0000 [thread overview]
Message-ID: <Zei7-0RMCpiWw49e@arm.com> (raw)
In-Reply-To: <20240122-arm64-sve-trap-mitigation-v4-1-54e0d78a3ae9@kernel.org>
On Mon, Jan 22, 2024 at 07:42:14PM +0000, Mark Brown wrote:
> This indicates that there should be some useful benefit from reducing the
> number of SVE access traps for blocking system calls like we did for non
> blocking system calls in commit 8c845e273104 ("arm64/sve: Leave SVE enabled
> on syscall if we don't context switch"). Let's do this by counting the
> number of times we have loaded FPSIMD only register state for SVE tasks
> and only disabling traps after some number of times, otherwise leaving
> traps disabled and flushing the non-shared register state like we would on
> trap.
It looks like some people complain about SVE being disabled, though I
assume this is for kernels prior to 6.2 and commit 8c845e273104
("arm64/sve: Leave SVE enabled on syscall if we don't context switch"):
https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1999551/comments/52
I assume we may see the other camp complaining about the additional
state saving on context switch.
Anyway, I don't see why we should treat blocking syscalls differently
from non-blocking ones (addressed by the commit above). I guess the
difference is that one goes through a context switch but, from a user
perspective, it's still a syscall. The SVE state is expected to be
discarded and there may be a preference for avoiding the subsequent
fault.
> I pulled 64 out of thin air for the number of flushes to do, there is
> doubtless room for tuning here. Ideally we would be able to tell if the
> task is actually using SVE but without using performance counters (which
> would be substantial work) we can't currently tell. I picked the number
> because so many of the tasks using SVE used it so frequently.
So I wonder whether we should make the timeout disabling behaviour the
same for both blocking and non-blocking syscalls. IOW, ignore the
context switching aspect. Every X number of returns, disable SVE
irrespective of whether it was context switched or not. Or, if the
number of returns has a variation in time, just use a jiffy (or other
time based figure), it would time out in the same way for all types of
workloads.
--
Catalin
next prev parent reply other threads:[~2024-03-06 18:54 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-22 19:42 [PATCH v4] arm64/fpsimd: Suppress SVE access traps when loading FPSIMD state Mark Brown
2024-03-06 18:54 ` Catalin Marinas [this message]
2024-03-06 22:44 ` Mark Brown
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