lkml.org 
[lkml]   [2013]   [Aug]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH 00/12] KVM: MMU: locklessly wirte-protect
On Sat, Aug 03, 2013 at 02:09:43PM +0900, Takuya Yoshikawa wrote:
> On Tue, 30 Jul 2013 21:01:58 +0800
> Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> wrote:
>
> > Background
> > ==========
> > Currently, when mark memslot dirty logged or get dirty page, we need to
> > write-protect large guest memory, it is the heavy work, especially, we need to
> > hold mmu-lock which is also required by vcpu to fix its page table fault and
> > mmu-notifier when host page is being changed. In the extreme cpu / memory used
> > guest, it becomes a scalability issue.
> >
> > This patchset introduces a way to locklessly write-protect guest memory.
>
> Nice improvements!
>
> If I read the patch set correctly, this work contains the following changes:
>
> Cleanups:
> Patch 1 and patch 12.
>
Yes, do not see the reason to not apply 1 straightaway. 12 depends on
other patches though.

> Lazy large page dropping for dirty logging:
> Patch 2-3.
> Patch 2 is preparatory to patch 3.
>
> This does not look like an RFC if you address Marcelo's comment.
> Any reason to include this in an RFC patch set?
Agree, you can post them separately for faster inclusion.

>
> Making remote TLBs flushable outside of mmu_lock for dirty logging:
> Patch 6.
>
> This is nice. I'm locally using a similar patch for my work, but yours
> is much cleaner and better. I hope this will get merged soon.
>
But without other patches this patch itself doesn't do much, no?

> New Pte-list handling:
> Patch 7-9.
>
> Still reading the details.
>
> RCU-based lockless write protection.
> Patch 10-11.
>
> If I understand RCU correctly, the current implementation has a problem:
> read-side critical sections can become too long.
>
> See the following LWN's article:
> "Sleepable RCU"
> https://lwn.net/Articles/202847/
>
> Especially, kvm_mmu_slot_remove_write_access() can take hundreds of
> milliseconds, or even a few seconds for guests using shadow paging.
> Is it possible to break the read-side critical section after protecting
> some pages? -- I guess so.
>
> Anyway, I want to see the following non-RFC quality patches get merged first:
> - Lazy large page dropping for dirty logging:
> - Making remote TLBs flushable outside of mmu_lock for dirty logging
>
> As you are doing in patch 11, the latter can eliminate the TLB flushes before
> cond_resched_lock(). So this alone is an optimization, and since my work is
> based on this TLB flush-less lock breaking, I would appriciate if you make this
> change first in your clean way.
>
> The remaining patches, pte-list refactoring and lock-less ones, also look
> interesting, but I need to read more to understand them.
>
> Thanks for the nice work!
Indeed. FWIW I completed the review and am waiting for a new version.

--
Gleb.


\
 
 \ /
  Last update: 2013-08-30 04:41    [W:0.288 / U:0.236 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site