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

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Message-Id: <20120503231107.e8c5a5dde90e109e570ba32e@gmail.com>
Date:	Thu, 3 May 2012 23:11:07 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Avi Kivity <avi@...hat.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 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.

I should have think of a way not to hold the spin_lock so long as Peter
said.  My lack of thinking might be the real problem.

Thanks,
	Takuya
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