linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Ben Gardon <bgardon@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Peter Xu <peterx@redhat.com>, Peter Shier <pshier@google.com>,
	Peter Feiner <pfeiner@google.com>,
	Junaid Shahid <junaids@google.com>,
	Jim Mattson <jmattson@google.com>,
	Yulei Zhang <yulei.kernel@gmail.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond_resched and needbreak
Date: Tue, 26 Jan 2021 09:47:19 -0800	[thread overview]
Message-ID: <CANgfPd_WvXP=mOnxFR8BY=WnbR5Gn8RpK7aR_mOrdDiCh4VEeQ@mail.gmail.com> (raw)
In-Reply-To: <460d38b9-d920-9339-1293-5900d242db37@redhat.com>

On Tue, Jan 26, 2021 at 6:38 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 21/01/21 01:19, Sean Christopherson wrote:
> > What if we simply make the common mmu_lock a union? The rwlock_t is
> > probably a bit bigger, but that's a few bytes for an entire VM. And
> > maybe this would entice/inspire other architectures to move to a similar
> > MMU model.
>
> Looking more at this, there is a problem in that MMU notifier functions
> take the MMU lock.
>
> Yes, qrwlock the size is a bit larger than qspinlock.  However, the fast
> path of qrwlocks is small, and if the slow paths are tiny compared to
> the mmu_lock critical sections that are so big as to require
> cond_resched.  So I would consider just changing all architectures to an
> rwlock.

I like the idea of putting the MMU lock union directly in struct KVM
and will make that change in the next version of this series. In my
testing, I found that wholesale replacing the spin lock with an rwlock
had a noticeable negative performance impact on the legacy / shadow
MMU. 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.

>
> Paolo
>

  reply	other threads:[~2021-01-27  0:37 UTC|newest]

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-12 18:10 [PATCH 00/24] Allow parallel page faults with TDP MMU Ben Gardon
2021-01-12 18:10 ` [PATCH 01/24] locking/rwlocks: Add contention detection for rwlocks Ben Gardon
2021-01-12 18:10 ` [PATCH 02/24] sched: Add needbreak " Ben Gardon
2021-01-12 18:10 ` [PATCH 03/24] sched: Add cond_resched_rwlock Ben Gardon
2021-01-12 18:10 ` [PATCH 04/24] kvm: x86/mmu: change TDP MMU yield function returns to match cond_resched Ben Gardon
2021-01-20 18:38   ` Sean Christopherson
2021-01-21 20:22     ` Paolo Bonzini
2021-01-26 14:11     ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 05/24] kvm: x86/mmu: Fix yielding in TDP MMU Ben Gardon
2021-01-20 19:28   ` Sean Christopherson
2021-01-22  1:06     ` Ben Gardon
2021-01-12 18:10 ` [PATCH 06/24] kvm: x86/mmu: Skip no-op changes in TDP MMU functions Ben Gardon
2021-01-20 19:51   ` Sean Christopherson
2021-01-25 23:51     ` Ben Gardon
2021-01-12 18:10 ` [PATCH 07/24] kvm: x86/mmu: Add comment on __tdp_mmu_set_spte Ben Gardon
2021-01-26 14:13   ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 08/24] kvm: x86/mmu: Add lockdep when setting a TDP MMU SPTE Ben Gardon
2021-01-20 19:58   ` Sean Christopherson
2021-01-26 14:13   ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 09/24] kvm: x86/mmu: Don't redundantly clear TDP MMU pt memory Ben Gardon
2021-01-20 20:06   ` Sean Christopherson
2021-01-26 14:14   ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 10/24] kvm: x86/mmu: Factor out handle disconnected pt Ben Gardon
2021-01-20 20:30   ` Sean Christopherson
2021-01-26 14:14   ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 11/24] kvm: x86/mmu: Put TDP MMU PT walks in RCU read-critical section Ben Gardon
2021-01-20 22:19   ` Sean Christopherson
2021-01-12 18:10 ` [PATCH 12/24] kvm: x86/kvm: RCU dereference tdp mmu page table links Ben Gardon
2021-01-22 18:32   ` Sean Christopherson
2021-01-26 18:17     ` Ben Gardon
2021-01-12 18:10 ` [PATCH 13/24] kvm: x86/mmu: Only free tdp_mmu pages after a grace period Ben Gardon
2021-01-12 18:10 ` [PATCH 14/24] kvm: mmu: Wrap mmu_lock lock / unlock in a function Ben Gardon
2021-01-13  2:35   ` kernel test robot
2021-01-12 18:10 ` [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond_resched and needbreak Ben Gardon
2021-01-21  0:19   ` Sean Christopherson
2021-01-21 20:17     ` Paolo Bonzini
2021-01-26 14:38     ` Paolo Bonzini
2021-01-26 17:47       ` Ben Gardon [this message]
2021-01-26 17:55         ` Paolo Bonzini
2021-01-26 18:11           ` Ben Gardon
2021-01-26 20:47             ` Paolo Bonzini
2021-01-27 20:08               ` Ben Gardon
2021-01-27 20:55                 ` Paolo Bonzini
2021-01-27 21:20                   ` Ben Gardon
2021-01-28  8:18                     ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 16/24] kvm: mmu: Wrap mmu_lock assertions Ben Gardon
2021-01-26 14:29   ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 17/24] kvm: mmu: Move mmu_lock to struct kvm_arch Ben Gardon
2021-01-12 18:10 ` [PATCH 18/24] kvm: x86/mmu: Use an rwlock for the x86 TDP MMU Ben Gardon
2021-01-21  0:45   ` Sean Christopherson
2021-01-12 18:10 ` [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock Ben Gardon
2021-01-21 19:22   ` Sean Christopherson
2021-01-21 21:32     ` Sean Christopherson
2021-01-26 14:27       ` Paolo Bonzini
2021-01-26 21:47         ` Ben Gardon
2021-01-26 22:02         ` Sean Christopherson
2021-01-26 22:09           ` Sean Christopherson
2021-01-27 12:40           ` Paolo Bonzini
2021-01-26 13:37   ` Paolo Bonzini
2021-01-26 21:07     ` Ben Gardon
2021-01-12 18:10 ` [PATCH 20/24] kvm: x86/mmu: Add atomic option for setting SPTEs Ben Gardon
2021-01-13  0:05   ` kernel test robot
2021-01-26 14:21   ` Paolo Bonzini
2021-01-12 18:10 ` [PATCH 21/24] kvm: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map Ben Gardon
2021-01-12 18:10 ` [PATCH 22/24] kvm: x86/mmu: Flush TLBs after zap in TDP MMU PF handler Ben Gardon
2021-01-21  0:05   ` Sean Christopherson
2021-01-12 18:10 ` [PATCH 23/24] kvm: x86/mmu: Freeze SPTEs in disconnected pages Ben Gardon
2021-01-12 18:10 ` [PATCH 24/24] kvm: x86/mmu: Allow parallel page faults for the TDP MMU Ben Gardon
2021-01-21  0:55   ` Sean Christopherson
2021-01-26 21:57     ` Ben Gardon
2021-01-27 17:14       ` Sean Christopherson
2021-01-26 13:37   ` Paolo Bonzini

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CANgfPd_WvXP=mOnxFR8BY=WnbR5Gn8RpK7aR_mOrdDiCh4VEeQ@mail.gmail.com' \
    --to=bgardon@google.com \
    --cc=jmattson@google.com \
    --cc=junaids@google.com \
    --cc=kernellwp@gmail.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=pbonzini@redhat.com \
    --cc=peterx@redhat.com \
    --cc=pfeiner@google.com \
    --cc=pshier@google.com \
    --cc=seanjc@google.com \
    --cc=vkuznets@redhat.com \
    --cc=xiaoguangrong.eric@gmail.com \
    --cc=yulei.kernel@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).