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

[Eric Dumazet <eric dumazet gmail com>]

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 ?

Really you shouldnt use rwlock in a path if this might hurt performance.

RCU is probably a better answer.

(bdev->bd_block_size should be read exactly once )