On Thu, Apr 28, 2016 at 11:58 PM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > On Thu, 28 Apr 2016, Ming Lei wrote: > >> Hi Mikulas, >> >> On Thu, Apr 28, 2016 at 11:29 PM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >> > >> > >> > On Thu, 28 Apr 2016, Ming Lei wrote: >> > >> >> There were reports about heavy stack use by recursive calling >> >> .bi_end_io()([1][2][3]). For example, more than 16K stack is >> >> consumed in a single bio complete path[3], and in [2] stack >> >> overflow can be triggered if 20 nested dm-crypt is used. >> >> >> >> Also patches[1] [2] [3] were posted for addressing the issue, >> >> but never be merged. And the idea in these patches is basically >> >> similar, all serializes the recursive calling of .bi_end_io() by >> >> percpu list. >> >> >> >> This patch still takes the same idea, but uses bio_list to >> >> implement it, which turns out more simple and the code becomes >> >> more readable meantime. >> >> >> >> One corner case which wasn't covered before is that >> >> bi_endio() may be scheduled to run in process context(such >> >> as btrfs), and this patch just bypasses the optimizing for >> >> that case because one new context should have enough stack space, >> >> and this approach isn't capable of optimizing it too because >> >> there isn't easy way to get a per-task linked list head. >> > >> > Hi >> > >> > You could use preempt_disable() and then you could use per-cpu list even >> > in the process context. >> >> Image why the .bi_end_io() is scheduled to process context, and the only >> workable/simple way I thought of is to use per-task list because it may sleep. > > The bi_end_io callback should not sleep, even if it is called from the > process context. If it shouldn't sleep, why is it scheduled to run in process context by paying extra context switch cost? And you can find that btrfs_subio_endio_read() does sleep for checksum stuff. Thanks, Ming > >> Given new context should have enough stack and only btrfs has this kind of >> usage as far as I see, so don't think that is worth of the optimization. >> >> Thanks, >> Ming > > Mikulas > -- > To unsubscribe from this list: send the line "unsubscribe linux-block" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
