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

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Message-ID: <4FA295BE.7010706@redhat.com>
Date:	Thu, 03 May 2012 17:27:10 +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 05:11 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.


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

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