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From:   Mingwei Zhang <mizhang@google.com>
Date:   Wed, 6 Apr 2022 10:53:23 -0700
Message-ID: <CAL715WLhy7EkJCyO7vzak3O8iw8GDRHkPF8aRtDedPXO1vx_Qw@mail.gmail.com>
Subject: Re: [PATCH] KVM: SEV: Add cond_resched() to loop in sev_clflush_pages()
To:     Sean Christopherson <seanjc@google.com>
Cc:     Peter Gonda <pgonda@google.com>, kvm <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Hi Sean,

> > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > > index 75fa6dd268f0..c2fe89ecdb2d 100644
> > > --- a/arch/x86/kvm/svm/sev.c
> > > +++ b/arch/x86/kvm/svm/sev.c
> > > @@ -465,6 +465,7 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
> > >                 page_virtual = kmap_atomic(pages[i]);
> > >                 clflush_cache_range(page_virtual, PAGE_SIZE);
> > >                 kunmap_atomic(page_virtual);
> > > +               cond_resched();
> >
> > If you add cond_resched() here, the frequency (once per 4K) might be
> > too high. You may want to do it once per X pages, where X could be
> > something like 1G/4K?
>
> No, every iteration is perfectly ok.  The "cond"itional part means that this will
> reschedule if and only if it actually needs to be rescheduled, e.g. if the task's
> timeslice as expired.  The check for a needed reschedule is cheap, using
> cond_resched() in tight-ish loops is ok and intended, e.g. KVM does a reched
> check prior to enterring the guest.

Double check on the code again. I think the point is not about flag
checking. Obviously branch prediction could really help. The point I
think is the 'call' to cond_resched(). Depending on the kernel
configuration, cond_resched() may not always be inlined, at least this
is my understanding so far? So if that is true, then it still might
not always be the best to call cond_resched() that often.