linux-kernel - Re: [PATCH 03/10] smp,cpumask: introduce on_each_cpu_cond

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Date:   Sun, 29 Jul 2018 13:51:13 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>, will.daecon@....com,
        Catalin Marinas <catalin.marinas@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 03/10] smp,cpumask: introduce on_each_cpu_cond_mask

On Sun, 2018-07-29 at 08:36 -0700, Andy Lutomirski wrote:
> On Jul 29, 2018, at 5:00 AM, Rik van Riel <riel@...riel.com> wrote:
> 
> > 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@...riel.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.
> > 
> > The test_func callbacks are not run remotely, but on
> > the local CPU, before deciding who to send callbacks
> > to.
> > 
> > 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).
> > 
> 
> What I meant is that on_each_cpu_mask does:
> 
> 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);
> }
> 
> 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 = 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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