Re: [RFC] [PATCH] A clean aEvgeniy pproach to writeout throttling

Jens Axboe
2007-12-10T13:22:19Z

On Mon, Dec 10 2007, Daniel Phillips wrote:
> On Monday 10 December 2007 04:32, Jens Axboe wrote:
> > I honestly don't know how to make this any clearer than I already did
> > above.
>
> Sure you do, you could cut out the rhetoric and save lots of bandwidth
> thereby.

I spent 3 mail explaining it as clearly as I could. So you're welcome
for the review and the reminder of why it's impossible to have a normal
conversation with you.

> Yes, the q = bdev_get_queue(bio->bi_bdev) needs to be repeated inside
> the submission loop, that was a flaw, thanks for the catch.

Precisely. So forgive me for thinking this patch hasn't seen very varied
testing, that's 2 errors (one simple, one bad - broken was NOT a gross
exageration, thanks) in very few lines.

>
> Regards,
>
> Daniel

> --- 2.6.24-rc3-mm.clean/block/ll_rw_blk.c 2007-12-04 14:45:25.000000000 -0800
> +++ 2.6.24-rc3-mm/block/ll_rw_blk.c 2007-12-10 04:49:56.000000000 -0800
> @@ -3210,9 +3210,9 @@ static inline int bio_check_eod(struct b
> */
> static inline void __generic_make_request(struct bio *bio)
> {
> - struct request_queue *q;
> + struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> sector_t old_sector;
> - int ret, nr_sectors = bio_sectors(bio);
> + int nr_sectors = bio_sectors(bio);
> dev_t old_dev;
> int err = -EIO;
>
> @@ -3221,6 +3221,13 @@ static inline void __generic_make_reques
> if (bio_check_eod(bio, nr_sectors))
> goto end_io;
>
> + if (q && q->metric && !bio->bi_queue) {
> + int need = bio->bi_throttle = q->metric(bio);
> + bio->bi_queue = q;
> + /* FIXME: potential race if atomic_sub is called in the middle of condition check */
> + wait_event(q->throttle_wait, atomic_read(&q->available) >= need);
> + atomic_sub(need, &q->available);
> + }
> /*
> * Resolve the mapping until finished. (drivers are
> * still free to implement/resolve their own stacking
> @@ -3231,10 +3238,9 @@ static inline void __generic_make_reques
> */
> old_sector = -1;
> old_dev = 0;
> - do {
> + while (1) {
> char b[BDEVNAME_SIZE];
>
> - q = bdev_get_queue(bio->bi_bdev);
> if (!q) {
> printk(KERN_ERR
> "generic_make_request: Trying to access "
> @@ -3282,8 +3288,10 @@ end_io:
> goto end_io;
> }
>
> - ret = q->make_request_fn(q, bio);
> - } while (ret);
> + if (!q->make_request_fn(q, bio))
> + return;
> + q = bdev_get_queue(bio->bi_bdev);
> + }

break here please.

> --- 2.6.24-rc3-mm.clean/include/linux/bio.h 2007-12-04 14:39:31.000000000 -0800
> +++ 2.6.24-rc3-mm/include/linux/bio.h 2007-12-04 23:31:41.000000000 -0800
> @@ -111,6 +111,9 @@ struct bio {
> bio_end_io_t *bi_end_io;
> atomic_t bi_cnt; /* pin count */
>
> + struct request_queue *bi_queue; /* for throttling */
> + unsigned bi_throttle; /* throttle metric */
> +

I still wish there was a way around this, you are bloating the bio by
about 15% (yeah I know you rambled on about this, but still). Better
placement would help, so there's still low hanging fruit available.

--
Jens Axboe