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Re: [dm-devel] [PATCH 2/3] Introduce percpu rw semaphores




On Sat, 28 Jul 2012, Eric Dumazet wrote:

> On Sat, 2012-07-28 at 12:41 -0400, Mikulas Patocka wrote:
> > Introduce percpu rw semaphores
> > 
> > When many CPUs are locking a rw semaphore for read concurrently, cache
> > line bouncing occurs. When a CPU acquires rw semaphore for read, the
> > CPU writes to the cache line holding the semaphore. Consequently, the
> > cache line is being moved between CPUs and this slows down semaphore
> > acquisition.
> > 
> > This patch introduces new percpu rw semaphores. They are functionally
> > identical to existing rw semaphores, but locking the percpu rw semaphore
> > for read is faster and locking for write is slower.
> > 
> > The percpu rw semaphore is implemented as a percpu array of rw
> > semaphores, each semaphore for one CPU. When some thread needs to lock
> > the semaphore for read, only semaphore on the current CPU is locked for
> > read. When some thread needs to lock the semaphore for write, semaphores
> > for all CPUs are locked for write. This avoids cache line bouncing.
> > 
> > Note that the thread that is locking percpu rw semaphore may be
> > rescheduled, it doesn't cause bug, but cache line bouncing occurs in
> > this case.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka redhat com>
> 
> I am curious to see how this performs with 4096 cpus ?

Each cpu should have its own rw semaphore in its cache, so I don't see a 
problem there.

When you change block size, all 4096 rw semaphores are locked for write, 
but changing block size is not a performance sensitive operation.

> Really you shouldnt use rwlock in a path if this might hurt performance.
> 
> RCU is probably a better answer.

RCU is meaningless here. RCU allows lockless dereference of a pointer. 
Here the problem is not pointer dereference, the problem is that integer 
bd_block_size may change.

> (bdev->bd_block_size should be read exactly once )

Rewrite all direct and non-direct io code so that it reads block size just 
once ...

Mikulas


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