linux-kernel - Re: [PATCH 4/5] arm64: mm: Support ASID isolation feature

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Message-ID: <Y2ughyNLh9EFw3HT@arm.com>
Date:   Wed, 9 Nov 2022 12:43:51 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     will@...nel.org, wangkefeng.wang@...wei.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linfeilong@...wei.com
Subject: Re: [PATCH 4/5] arm64: mm: Support ASID isolation feature

On Mon, Oct 17, 2022 at 04:32:02PM +0800, Yunfeng Ye wrote:
> After a rollover, the global generation will be flushed, which will
> cause the process mm->context.id on all CPUs do not match the
> generation. Thus, the process will compete for the global spinlock lock
> to reallocate a new ASID and refresh the TLBs of all CPUs on context
> switch. This will lead to the increase of scheduling delay and TLB miss.
> 
> In some delay-sensitive scenarios, for example, part of CPUs are
> isolated, only a limited number of processes are deployed to run on the
> isolated CPUs. In this case, we do not want these key processes to be
> affected by the rollover of ASID.

Part of this commit log should also go in the cover letter and it would
help to back this up by some numbers, e.g. what percentage improvement
you get with this patchset by running hackbench on an isolated CPU.

In theory it looks like CPU isolation would benefit from this patchset
but we try not to touch this code often, so any modification should come
with proper justification, backed by numbers.

Note that I haven't reviewed the algorithm you are proposing in detail,
only had a brief look.

-- 
Catalin