On Wed, 24 Oct 2012, Dave Chinner wrote:
> On Fri, Oct 19, 2012 at 06:54:41PM -0400, Mikulas Patocka wrote: > > > > > > On Fri, 19 Oct 2012, Peter Zijlstra wrote: > > > > > > Yes, I tried this approach - it involves doing LOCK instruction on read > > > > lock, remembering the cpu and doing another LOCK instruction on read > > > > unlock (which will hopefully be on the same CPU, so no cacheline > > > > bouncing > > > > happens in the common case). It was slower than the approach without > > > > any > > > > LOCK instructions (43.3 seconds seconds for the implementation with > > > > per-cpu LOCKed access, 42.7 seconds for this implementation without > > > > atomic > > > > instruction; the benchmark involved doing 512-byte direct-io reads and > > > > writes on a ramdisk with 8 processes on 8-core machine). > > > > > > So why is that a problem? Surely that's already tons better then what > > > you've currently got. > > > > Percpu rw-semaphores do not improve performance at all. I put them there > > to avoid performance regression, not to improve performance. > > > > All Linux kernels have a race condition - when you change block size of a > > block device and you read or write the device at the same time, a crash > > may happen. This bug is there since ever. Recently, this bug started to > > cause major trouble - multiple high profile business sites report crashes > > because of this race condition. > > > > You can fix this race by using a read lock around I/O paths and write lock > > around block size changing, but normal rw semaphore cause cache line > > bouncing when taken for read by multiple processors and I/O performance > > degradation because of it is measurable. > > This doesn't sound like a new problem. Hasn't this global access, > single modifier exclusion problem been solved before in the VFS? > e.g. mnt_want_write()/mnt_make_readonly() > > Cheers, > > Dave. Yes, mnt_want_write()/mnt_make_readonly() do the same thing as percpu rw semaphores. I think you can convert mnt_want_write()/mnt_make_readonly() to use percpu rw semaphores and remove the duplicated code. Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
