Re: [RFC] sched: make callers check lock contention for cond_resched_lock()

Avi Kivity
2012-05-03T14:27:30Z

On 05/03/2012 05:11 PM, Takuya Yoshikawa wrote:
> On Thu, 03 May 2012 15:47:26 +0300
> Avi Kivity <avi@redhat.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.

--
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