linux-kernel - Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond

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Message-ID: <CANgfPd_TOpc_cinPwAyH-0WajRM1nZvn9q6s70jno5LFf2vsdQ@mail.gmail.com>
Date:   Tue, 26 Jan 2021 10:11:58 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond_resched and needbreak

On Tue, Jan 26, 2021 at 9:55 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 26/01/21 18:47, Ben Gardon wrote:
> > Enough that it motivated me to implement this more complex union
> > scheme. While the difference was pronounced in the dirty log perf test
> > microbenchmark, it's an open question as to whether it would matter in
> > practice.
>
> I'll look at getting some numbers if it's just the dirty log perf test.
>   Did you see anything in the profile pointing specifically at rwlock?

When I did a strict replacement I found ~10% worse memory population
performance.
Running dirty_log_perf_test -v 96 -b 3g -i 5 with the TDP MMU
disabled, I got 119 sec to populate memory as the baseline and 134 sec
with an earlier version of this series which just replaced the
spinlock with an rwlock. I believe this difference is statistically
significant, but didn't run multiple trials.
I didn't take notes when profiling, but I'm pretty sure the rwlock
slowpath showed up a lot. This was a very high contention scenario, so
it's probably not indicative of real-world performance.
In the slow path, the rwlock is certainly slower than a spin lock.

If the real impact doesn't seem too large, I'd be very happy to just
replace the spinlock.

>
> Paolo
>