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

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Date:   Wed, 27 Jan 2021 12:08:19 -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 12:48 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 26/01/21 19:11, Ben Gardon wrote:
> > 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.
>
> Ok, so let's use the union idea and add a "#define KVM_HAVE_MMU_RWLOCK"
> to x86.  The virt/kvm/kvm_main.c MMU notifiers functions can use the
> #define to pick between write_lock and spin_lock.

I'm not entirely sure I understand this suggestion. Are you suggesting
we'd have the spinlock and rwlock in a union in struct kvm but then
use a static define to choose which one is used by other functions? It
seems like if we're using static defines the union doesn't add value.
If we do use the union, I think the advantages offered by __weak
wrapper functions, overridden on a per-arch basis, are worthwhile.

>
> For x86 I want to switch to tdp_mmu=1 by default as soon as parallel
> page faults are in, so we can use the rwlock unconditionally and drop
> the wrappers, except possibly for some kind of kvm_mmu_lock/unlock_root
> that choose between read_lock for TDP MMU and write_lock for shadow MMU.
>
> Thanks!
>
> Paolo
>