linux-kernel - Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87jze9rq15.fsf@oracle.com>
Date: Tue, 15 Oct 2024 14:53:58 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
        Ankur Arora
 <ankur.a.arora@...cle.com>, linux-pm@...r.kernel.org,
        kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, will@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, pbonzini@...hat.com,
        wanpengli@...cent.com, vkuznets@...hat.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, peterz@...radead.org, arnd@...db.de,
        lenb@...nel.org, mark.rutland@....com, harisokn@...zon.com,
        mtosatti@...hat.com, sudeep.holla@....com, misono.tomohiro@...itsu.com,
        maobibo@...ngson.cn, joao.m.martins@...cle.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v8 01/11] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()


Catalin Marinas <catalin.marinas@....com> writes:

> On Tue, Oct 15, 2024 at 10:17:13AM -0700, Christoph Lameter (Ampere) wrote:
>> On Tue, 15 Oct 2024, Catalin Marinas wrote:
>> > > Setting of need_resched() from another processor involves sending an IPI
>> > > after that was set. I dont think we need to smp_cond_load_relaxed since
>> > > the IPI will cause an event. For ARM a WFE would be sufficient.
>> >
>> > I'm not worried about the need_resched() case, even without an IPI it
>> > would still work.
>> >
>> > The loop_count++ side of the condition is supposed to timeout in the
>> > absence of a need_resched() event. You can't do an smp_cond_load_*() on
>> > a variable that's only updated by the waiting CPU. Nothing guarantees to
>> > wake it up to update the variable (the event stream on arm64, yes, but
>> > that's generic code).
>>
>> Hmm... I have WFET implementation here without smp_cond modelled after
>> the delay() implementation ARM64 (but its not generic and there is
>> an additional patch required to make this work. Intermediate patch
>> attached)
>
> At least one additional patch ;). But yeah, I suggested hiding all this
> behind something like smp_cond_load_timeout() which would wait on
> current_thread_info()->flags but with a timeout. The arm64
> implementation would follow some of the logic in __delay(). Others may
> simply poll with cpu_relax().
>
> Alternatively, if we get an IPI anyway, we can avoid smp_cond_load() and
> rely on need_resched() and some new delay/cpu_relax() API that waits for
> a timeout or an IPI, whichever comes first. E.g. cpu_relax_timeout()
> which on arm64 it's just a simplified version of __delay() without the
> 'while' loops.

AFAICT when polling (which we are since poll_idle() calls
current_set_polling_and_test()), the scheduler will elide the IPI
by remotely setting the need-resched bit via set_nr_if_polling().

Once we stop polling then the scheduler should take the IPI path
because call_function_single_prep_ipi() will fail.

--
ankur