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Message-ID: <1532886673.28585.24.camel@surriel.com>
Subject: Re: [PATCH 03/10] smp,cpumask: introduce on_each_cpu_cond_mask
From:   Rik van Riel <riel@surriel.com>
To:     Andy Lutomirski <luto@amacapital.net>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Mike Galbraith <efault@gmx.de>,
        Dave Hansen <dave.hansen@intel.com>, will.daecon@arm.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date:   Sun, 29 Jul 2018 13:51:13 -0400
In-Reply-To: <E710FBA5-CC5E-4941-ACBF-4AB3424F1F68@amacapital.net>
References: <20180728215357.3249-1-riel@surriel.com>
         <20180728215357.3249-4-riel@surriel.com>
         <CALCETrUipqat-78++E8fL7d2=77Cbj72jCTZMO=yNF8nX9HhCA@mail.gmail.com>
         <1532865634.28585.2.camel@surriel.com>
         <E710FBA5-CC5E-4941-ACBF-4AB3424F1F68@amacapital.net>
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On Sun, 2018-07-29 at 08:36 -0700, Andy Lutomirski wrote:
> On Jul 29, 2018, at 5:00 AM, Rik van Riel <riel@surriel.com> wrote:
>=20
> > On Sat, 2018-07-28 at 19:57 -0700, Andy Lutomirski wrote:
> > > On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@surriel.com>
> > > wrote:
> > > > Introduce a variant of on_each_cpu_cond that iterates only over
> > > > the
> > > > CPUs in a cpumask, in order to avoid making callbacks for every
> > > > single
> > > > CPU in the system when we only need to test a subset.
> > > Nice.
> > > Although, if you want to be really fancy, you could optimize this
> > > (or
> > > add a variant) that does the callback on the local CPU in
> > > parallel
> > > with the remote ones.  That would give a small boost to TLB
> > > flushes.
> >=20
> > The test_func callbacks are not run remotely, but on
> > the local CPU, before deciding who to send callbacks
> > to.
> >=20
> > The actual IPIs are sent in parallel, if the cpumask
> > allocation succeeds (it always should in many kernel
> > configurations, and almost always in the rest).
> >=20
>=20
> What I meant is that on_each_cpu_mask does:
>=20
> smp_call_function_many(mask, func, info, wait);
> if (cpumask_test_cpu(cpu, mask)) {
>    unsigned long flags;
>    local_irq_save(flags); func(info);
>    local_irq_restore(flags);
> }
>=20
> So it IPIs all the remote CPUs in parallel, then waits, then does the
> local work.  In principle, the local flush could be done after
> triggering the IPIs but before they all finish.

Grepping around the code, I found a few examples where the
calling code appears to expect that smp_call_function_many
also calls "func" on the local CPU.

For example, kvm_emulate_wbinvd_noskip has this:

        if (kvm_x86_ops->has_wbinvd_exit()) {
                int cpu =3D get_cpu();

                cpumask_set_cpu(cpu, vcpu->arch.wbinvd_dirty_mask);
                smp_call_function_many(vcpu->arch.wbinvd_dirty_mask,
                                wbinvd_ipi, NULL, 1);
                put_cpu();
                cpumask_clear(vcpu->arch.wbinvd_dirty_mask);
        } else
                wbinvd();

This seems to result in systems with ->has_wbinvd_exit
only calling wbinvd_ipi on OTHER CPUs, and not on the
CPU where the guest exited with wbinvd?

This seems unintended.

I guess looking into on_each_cpu_mask might be a little
higher priority than waiting until the next Outreachy
season :)

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