Messages in this thread | | | Subject | Re: [Patch] fix MTD CFI/LPDDR flash driver huge latency bug | From | Stefani Seibold <> | Date | Sat, 13 Mar 2010 13:31:30 +0100 |
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Am Freitag, den 12.03.2010, 14:23 -0800 schrieb Andrew Morton: > On Sat, 06 Mar 2010 17:48:57 +0100 > Stefani Seibold <stefani@seibold.net> wrote: > > > This patch fix a huge latency problem in the MTD CFI and LPDDR flash > > drivers. > > > > The use of a memcpy() during a spinlock operation will cause very long > > thread context switch delays if the flash chip bandwidth is low and the > > data to be copied large, because a spinlock will disable preemption. > > > > For example: A flash with 6,5 MB/s bandwidth will cause under ubifs, > > which request sometimes 128 KB (the flash erase size), a preemption > > delay of 20 milliseconds. High priority threads will not be served > > during this time, regardless whether this threads access the flash or > > not. This behavior breaks real time. > > > > The patch change all the use of spin_lock operations for xxxx->mutex > > into mutex operations, which is exact what the name says and means. > > > > There is no performance regression since the mutex is normally not > > acquired. > > hm, big scary patch. Are you sure this mutex is never taken from > atomic or irq contexts? Is it ully tested with all relevant debug options > and lockdep enabled? > >
I have analyzed this drivers and IMHO i don't think there will be used from irq or atomic contexts. There is no request interrupt and there are a lot msleep and add_wait_queues/schedule calls during holding the mutex, which are not very useful in a irq or atomic context. But i don't know the whole mtd stack.
I tested the patch with the following kernel debug options:
CONFIG_DEBUG_KERNEL=y CONFIG_DETECT_SOFTLOCKUP=y CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC_VALUE=0 CONFIG_SCHED_DEBUG=y CONFIG_SCHEDSTATS=y CONFIG_TIMER_STATS=y CONFIG_DEBUG_MUTEXES=y CONFIG_DEBUG_SPINLOCK_SLEEP=y
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