linux-kernel - RE: [PATCH v5 0/6] Add hardware prefetch control driver for A64FX and x86

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Date:   Tue, 28 Jun 2022 16:39:31 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "tarumizu.kohei@...itsu.com" <tarumizu.kohei@...itsu.com>,
        'Linus Walleij' <linus.walleij@...aro.org>
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Subject: RE: [PATCH v5 0/6] Add hardware prefetch control driver for A64FX and
 x86

>> The right way to solve this is to make the Linux kernel contain the necessary
>> heuristics to identify which tasks and thus cores need this to improve efficiency
>> and then apply it automatically.
>>
>> Putting it in userspace is making a human do a machines job which isn't
>> sustainable.
>>
>> By putting the heuristics in kernelspace Linux will improve performance also on
>> workloads the human operator didn't think of as the machine will detect them from
>> statictical or other behaviour patterns.
>
>In order to put the heuristics into kernelspace Linux, I think it
>necessary to consider the following two points.
>
>1) Which cores are tied with the process?
>This is different from the core on which the process can run. It
>probably need to combine some CPU resource limit to avoid affecting
>non-target processes.
>
>2) How to derive the value to set in the register?
>It is necessary to verify whether an appropriate set value can be
>derived using statistical information, etc. In addition, to prevent
>the cost of automatic derivation from exceeding the value that would
>be improved by it.
>
>I don't have a prospect for resolving these issues yet. I will
>continue these considerations.

Another approach would be to make the set of prefetch settings
a task attribute. Then set them in the context switch code when
the process is about to run on a CPU.

But that assumes you can cheaply change the attributes. If doing
so requires multiple MSR writes (on x86) it might be a non-starter.

-Tony