linux-kernel - Re: [PATCH v5 3/8] KVM: MMU: fast invalidate all pages

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Date:	Thu, 16 May 2013 15:14:35 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v5 3/8] KVM: MMU: fast invalidate all pages

Il 16/05/2013 14:43, Gleb Natapov ha scritto:
>> > +restart:
>> > +	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
>> > +		if (!is_obsolete_sp(kvm, sp))
>> > +			continue;
> What if we save kvm->arch.active_mmu_pages on the stack and init
> kvm->arch.active_mmu_pages to be empty at the entrance to
> zap_invalid_pages(). This loop will iterate over saved list. This will
> allow us to drop the is_obsolete_sp() check and will save time since we
> will not be iterating over newly created sps.
> 

But when you add cond_resched_lock a thread may want to zap pages itself
(e.g. from prepare_zap_oldest_mmu_page) and it won't find them.

Here is another proposal...  The idea is to avoid looking at new pages
more than necessary after a "goto restart".

Basically, you alternate between two phases:

- look for pages to be zapped, group them together

- zap the pages

Something like:

      moved = 0;
restart:
      zapping = true;
      for each page in active_mmu_pages [reverse and safe] {
             if (!is_obsolete || invalid) {
                 /*
                  * Found a new page, stop zapping for now and
                  * try to segregate the invalid ones at one end
                  * of the list.
                  */
                 zapping = false;
                 continue;
             }

             if (batch > 10 && ...) {
                 cond_resched_lock
                 batch = 0;
                 goto restart;
             }

             if (!zapping) {
                 /*
                  * Segregate pages to one end of the list where
                  * new pages don't get in the way.
                  */
                 list_move_tail(page, active_mmu_pages)
                 batch++; /* or maybe not? */
                 moved++;
             } else {
                 batch += prepare_zap_page
                 goto restart;
             }
     }

     /* Need another pass to look at segregated pages?  */
     if (moved) {
         moved = 0;
         goto restart;
     }
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