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

Avi Kivity
2012-05-03T14:38:23Z

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@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.
>

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