From: Mingwei Zhang <mizhang@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Peter Gonda <pgonda@google.com>, kvm <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KVM: SEV: Add cond_resched() to loop in sev_clflush_pages()
Date: Wed, 6 Apr 2022 10:53:23 -0700 [thread overview]
Message-ID: <CAL715WLhy7EkJCyO7vzak3O8iw8GDRHkPF8aRtDedPXO1vx_Qw@mail.gmail.com> (raw)
In-Reply-To: <YkYdlfYM/FWlMqMg@google.com>
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.
next prev parent reply other threads:[~2022-04-06 18:59 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-30 16:43 [PATCH] KVM: SEV: Add cond_resched() to loop in sev_clflush_pages() Peter Gonda
2022-03-30 16:52 ` Mingwei Zhang
2022-03-31 21:31 ` Sean Christopherson
2022-04-05 19:26 ` Peter Gonda
2022-04-06 17:53 ` Mingwei Zhang [this message]
2022-04-06 18:26 ` Sean Christopherson
2022-04-06 18:34 ` Peter Gonda
2022-04-18 15:48 ` Sean Christopherson
2022-04-21 14:35 ` Peter Gonda
2022-04-21 14:46 ` Sean Christopherson
2022-04-05 12:12 ` Paolo Bonzini
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