[PATCHv2 7/7] zram: cond_resched() in writeback loop

[Sergey Senozhatsky <senozhatsky@chromium.org>]

zram writeback is a costly operation, because every target slot
(unless ZRAM_HUGE) is decompressed before it gets written to a
backing device.  The writeback to a backing device uses
submit_bio_wait() which may look like a rescheduling point.
However, if the backing device has BD_HAS_SUBMIT_BIO bit set
__submit_bio() calls directly disk->fops->submit_bio(bio) on
the backing device and so when submit_bio_wait() calls
blk_wait_io() the I/O is already done.  On such systems we
effective end up in a loop

    for_each (target slot) {
	decompress(slot)
	__submit_bio()
	    disk->fops->submit_bio(bio)
    }

Which on PREEMPT_NONE systems triggers watchdogs (since there
are no explicit rescheduling points).  Add cond_resched() to
the zram writeback loop.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zram_drv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 7dd72b58e921..5b8e4f4171ab 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -884,6 +884,8 @@ static ssize_t writeback_store(struct device *dev,
 next:
 		zram_slot_unlock(zram, index);
 		release_pp_slot(zram, pps);
+
+		cond_resched();
 	}
 
 	if (blk_idx)
-- 
2.47.1.613.gc27f4b7a9f-goog