linux-kernel - Re: [RFC PATCH v3 11/15] context-tracking: Introduce work deferral infrastructure

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Message-ID: <xhsmh1pz39v0k.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Fri, 22 Nov 2024 15:56:59 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
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Subject: Re: [RFC PATCH v3 11/15] context-tracking: Introduce work deferral
 infrastructure

On 20/11/24 18:30, Frederic Weisbecker wrote:
> Le Wed, Nov 20, 2024 at 06:10:43PM +0100, Valentin Schneider a écrit :
>> On 20/11/24 15:23, Frederic Weisbecker wrote:
>>
>> > Ah but there is CT_STATE_GUEST and I see the last patch also applies that to
>> > CT_STATE_IDLE.
>> >
>> > So that could be:
>> >
>> > bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
>> > {
>> >    struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
>> >    unsigned int old;
>> >    bool ret = false;
>> >
>> >    preempt_disable();
>> >
>> >    old = atomic_read(&ct->state);
>> >
>> >    /* CT_STATE_IDLE can be added to last patch here */
>> >    if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) {
>> >            old &= ~CT_STATE_MASK;
>> >            old |= CT_STATE_USER;
>> >    }
>>
>> Hmph, so that lets us leverage the cmpxchg for a !CT_STATE_KERNEL check,
>> but we get an extra loop if the target CPU exits kernelspace not to
>> userspace (e.g. vcpu or idle) in the meantime - not great, not terrible.
>
> The thing is, what you read with atomic_read() should be close to reality.
> If it already is != CT_STATE_KERNEL then you're good (minus racy changes).
> If it is CT_STATE_KERNEL then you still must do a failing cmpxchg() in any case,
> at least to make sure you didn't miss a context tracking change. So the best
> you can do is a bet.
>
>>
>> At the cost of one extra bit for the CT_STATE area, with CT_STATE_KERNEL=1
>> we could do:
>>
>>   old = atomic_read(&ct->state);
>>   old &= ~CT_STATE_KERNEL;
>
> And perhaps also old |= CT_STATE_IDLE (I'm seeing the last patch now),
> so you at least get a chance of making it right (only ~CT_STATE_KERNEL
> will always fail) and CPUs usually spend most of their time idle.
>

I'm thinking with:

        CT_STATE_IDLE		= 0,
        CT_STATE_USER		= 1,
        CT_STATE_GUEST		= 2,
        CT_STATE_KERNEL		= 4, /* Keep that as a standalone bit */

we can stick with old &= ~CT_STATE_KERNEL; and that'll let the cmpxchg
succeed for any of IDLE/USER/GUEST.

> Thanks.