Messages in this thread | | | Subject | Re: [Patch] fix MTD CFI/LPDDR flash driver huge latency bug | From | David Woodhouse <> | Date | Fri, 19 Mar 2010 08:29:15 +0000 |
| |
On Mon, 2010-03-15 at 14:24 +0000, Jamie Lokier wrote: > > > > The problem is the memcpy operation which is very slow. A cond_resched > > > > wouldn't help, since the cpu bus is blocked during the transfer of the > > > > word. > > > > > > I mean split the memcpy into multiple smaller memcpys, so that the > > > total time in each memcpy is limited to something reasonable. > > > > > > The check in cond_resched() is fast, especially once cached. memcpy > > > speed depends a lot on the attached flash and how everything's > > > configured, varying from 2.5MB/s up to hundreds of MB/s. So how about > > > doing cond_resched() every 256 bytes? > > > > > > -- Jamie > > > > I thoght about this aporoach and i don't like this idea. Why not using a > > preemptible kernel? > > Because it introduces too many risks to enable CONFIG_PREEMPT in a > stable rolled out device which isn't using it already. Especially on > devices where it's not well tested by other people, and with drivers > that nobody ever used with CONFIG_PREEMPT before. > > And because CONFIG_PREEMPT isn't always better. (Why do you think > it's a config option?) > > As a bug fix for observed high scheduling latency when a flash I/O is > occurring, splitting the memcpys is a good choice. I will be trying > it on my kernels, even if it doesn't get mainlined. Thanks for the idea ;-)
Rather than pulling a number our of our posterior like "every 256 bytes" which might _really_ screw up performance of some architectures' memcpy routines, I suspect we might want the platform to provide an optimised "sleepable_memcpy" function which does it at whatever interval is appropriate for the memcpy routine in use. Or magically makes it preemptable. Or uses a DMA engine. Or whatever.
I wonder where else we could use such a function...
-- David Woodhouse Open Source Technology Centre David.Woodhouse@intel.com Intel Corporation
| |