Re: [PATCH v3 1/2] locking/qrwlock: Better optimization for interrupt context readers

[Waiman Long]

On 06/16/2015 08:17 AM, Will Deacon wrote:
> Hi Waiman,
>
> On Mon, Jun 15, 2015 at 11:24:02PM +0100, Waiman Long wrote:
> > The qrwlock is fair in the process context, but becoming unfair when
> > in the interrupt context to support use cases like the tasklist_lock.
> >
> > The current code isn't that well-documented on what happens when
> > in the interrupt context. The rspin_until_writer_unlock() will only
> > spin if the writer has gotten the lock. If the writer is still in the
> > waiting state, the increment in the reader count will cause the writer
> > to remain in the waiting state and the new interrupt context reader
> > will get the lock and return immediately. The current code, however,
> > do an additional read of the lock value which is not necessary as the
> > information have already been there in the fast path. This may sometime
> > cause an additional cacheline load when the lock is highly contended.
> >
> > This patch passes the lock value information gotten in the fast path
> > to the slow path to eliminate the additional read. It also clarify the
> > action for the interrupt context readers more explicitly.
> >
> > Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
> > ---
> >   include/asm-generic/qrwlock.h |    4 ++--
> >   kernel/locking/qrwlock.c      |   14 ++++++++------
> >   2 files changed, 10 insertions(+), 8 deletions(-)
> [...]
>
> > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> > index 00c12bb..d7d7557 100644
> > --- a/kernel/locking/qrwlock.c
> > +++ b/kernel/locking/qrwlock.c
> > @@ -43,22 +43,24 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
> >    * queue_read_lock_slowpath - acquire read lock of a queue rwlock
> >    * @lock: Pointer to queue rwlock structure
> >    */
> > -void queue_read_lock_slowpath(struct qrwlock *lock)
> > +void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
> >   {
> > -	u32 cnts;
> > -
> >   	/*
> >   	 * Readers come here when they cannot get the lock without waiting
> >   	 */
> >   	if (unlikely(in_interrupt())) {
> >   		/*
> > -		 * Readers in interrupt context will spin until the lock is
> > -		 * available without waiting in the queue.
> > +		 * Readers in interrupt context will get the lock immediately
> > +		 * if the writer is just waiting (not holding the lock yet)
> > +		 * or they will spin until the lock is available without
> > +		 * waiting in the queue.
> >   		 */
> > -		cnts = smp_load_acquire((u32 *)&lock->cnts);
> > +		if ((cnts&  _QW_WMASK) != _QW_LOCKED)
> > +			return;
> I really doubt the check here is gaining you any performance, given
> rspin_until_write_unlock does the same check immediately and should be
> inlined. Just dropping the acquire and passing cnts through should be
> sufficient.

Yes, you are right. I can just pass the cnt to 
rspin_until_write_unlock() and be done with it.

Cheers,
Longman
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