linux-kernel - Re: [RFC] sched: make callers check lock contention for cond_resched

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Message-ID: <4FA2985F.6080802@redhat.com>
Date:	Thu, 03 May 2012 17:38:23 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kvm@...r.kernel.org, mtosatti@...hat.com,
	yoshikawa.takuya@....ntt.co.jp
Subject: Re: [RFC] sched: make callers check lock contention for cond_resched_lock()

On 05/03/2012 05:27 PM, Avi Kivity wrote:
> On 05/03/2012 05:11 PM, Takuya Yoshikawa wrote:
> > On Thu, 03 May 2012 15:47:26 +0300
> > Avi Kivity <avi@...hat.com> wrote:
> >
> > > On 05/03/2012 03:29 PM, Peter Zijlstra wrote:
> > > > On Thu, 2012-05-03 at 21:22 +0900, Takuya Yoshikawa wrote:
> > > > > Although the real use case is out of this RFC patch, we are now discussing
> > > > > a case in which we may hold a spin_lock for long time, ms order, depending
> > > > > on workload;  and in that case, other threads -- VCPU threads -- should be
> > > > > given higher priority for that problematic lock. 
> > > >
> > > > Firstly, if you can hold a lock that long, it shouldn't be a spinlock,
> > > 
> > > In fact with your mm preemptibility work it can be made into a mutex, if
> > > the entire mmu notifier path can be done in task context.  However it
> > > ends up a strange mutex - you can sleep while holding it but you may not
> > > allocate, because you might recurse into an mmu notifier again.
> > > 
> > > Most uses of the lock only involve tweaking some bits though.
> >
> > I might find a real way to go.
> >
> > After your "mmu_lock -- TLB-flush" decoupling, we can change the current
> > get_dirty work flow like this:
> >
> > 	for ... {
> > 		take mmu_lock
> > 		for 4K*8 gfns {		// with 4KB dirty_bitmap_buffer
> > 			xchg dirty bits	// 64/32 gfns at once
> > 			write protect them
> > 		}
> > 		release mmu_lock
> > 		copy_to_user
> > 	}
> > 	TLB flush
> >
> > This reduces the size of dirty_bitmap_buffer and does not hold mmu_lock
> > so long.
>
> Good idea.  Hopefully the lock acquisition costs are low enough - we're
> adding two atomic operations per iteration here.
>

btw, this requires my kvm_cond_flush_remote_tlbs().  Otherwise another
thread can acquire the lock, see a pagetable marked read-only by this
code, and proceed to shadow it, while the guest still has a writeable
tlb entry pointing at it.

-- 
error compiling committee.c: too many arguments to function

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