From 796d5116c407690b14fd5bda136aa67a39e7061a Mon Sep 17 00:00:00 2001 From: Jeff Moyer Date: Thu, 2 Jun 2011 21:19:05 +0200 Subject: [PATCH] iosched: prevent aliased requests from starving other I/O Hi, Jens, If you recall, I posted an RFC patch for this back in July of last year: http://lkml.org/lkml/2010/7/13/279 The basic problem is that a process can issue a never-ending stream of async direct I/Os to the same sector on a device, thus starving out other I/O in the system (due to the way the alias handling works in both cfq and deadline). The solution I proposed back then was to start dispatching from the fifo after a certain number of aliases had been dispatched. Vivek asked why we had to treat aliases differently at all, and I never had a good answer. So, I put together a simple patch which allows aliases to be added to the rb tree (it adds them to the right, though that doesn't matter as the order isn't guaranteed anyway). I think this is the preferred solution, as it doesn't break up time slices in CFQ or batches in deadline. I've tested it, and it does solve the starvation issue. Let me know what you think. Cheers, Jeff Signed-off-by: Jeff Moyer Signed-off-by: Jens Axboe --- block/cfq-iosched.c | 9 ++------- block/deadline-iosched.c | 4 +--- block/elevator.c | 7 ++----- include/linux/elevator.h | 2 +- 4 files changed, 6 insertions(+), 16 deletions(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 7c52d6888924..a2fb14bb1402 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1501,16 +1501,11 @@ static void cfq_add_rq_rb(struct request *rq) { struct cfq_queue *cfqq = RQ_CFQQ(rq); struct cfq_data *cfqd = cfqq->cfqd; - struct request *__alias, *prev; + struct request *prev; cfqq->queued[rq_is_sync(rq)]++; - /* - * looks a little odd, but the first insert might return an alias. - * if that happens, put the alias on the dispatch list - */ - while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL) - cfq_dispatch_insert(cfqd->queue, __alias); + elv_rb_add(&cfqq->sort_list, rq); if (!cfq_cfqq_on_rr(cfqq)) cfq_add_cfqq_rr(cfqd, cfqq); diff --git a/block/deadline-iosched.c b/block/deadline-iosched.c index 5139c0ea1864..c644137d9cd6 100644 --- a/block/deadline-iosched.c +++ b/block/deadline-iosched.c @@ -77,10 +77,8 @@ static void deadline_add_rq_rb(struct deadline_data *dd, struct request *rq) { struct rb_root *root = deadline_rb_root(dd, rq); - struct request *__alias; - while (unlikely(__alias = elv_rb_add(root, rq))) - deadline_move_request(dd, __alias); + elv_rb_add(root, rq); } static inline void diff --git a/block/elevator.c b/block/elevator.c index b0b38ce0dcb6..a3b64bc71d88 100644 --- a/block/elevator.c +++ b/block/elevator.c @@ -353,7 +353,7 @@ static struct request *elv_rqhash_find(struct request_queue *q, sector_t offset) * RB-tree support functions for inserting/lookup/removal of requests * in a sorted RB tree. */ -struct request *elv_rb_add(struct rb_root *root, struct request *rq) +void elv_rb_add(struct rb_root *root, struct request *rq) { struct rb_node **p = &root->rb_node; struct rb_node *parent = NULL; @@ -365,15 +365,12 @@ struct request *elv_rb_add(struct rb_root *root, struct request *rq) if (blk_rq_pos(rq) < blk_rq_pos(__rq)) p = &(*p)->rb_left; - else if (blk_rq_pos(rq) > blk_rq_pos(__rq)) + else if (blk_rq_pos(rq) >= blk_rq_pos(__rq)) p = &(*p)->rb_right; - else - return __rq; } rb_link_node(&rq->rb_node, parent, p); rb_insert_color(&rq->rb_node, root); - return NULL; } EXPORT_SYMBOL(elv_rb_add); diff --git a/include/linux/elevator.h b/include/linux/elevator.h index 21a8ebf2dc3a..d800d5142184 100644 --- a/include/linux/elevator.h +++ b/include/linux/elevator.h @@ -146,7 +146,7 @@ extern struct request *elv_rb_latter_request(struct request_queue *, struct requ /* * rb support functions. */ -extern struct request *elv_rb_add(struct rb_root *, struct request *); +extern void elv_rb_add(struct rb_root *, struct request *); extern void elv_rb_del(struct rb_root *, struct request *); extern struct request *elv_rb_find(struct rb_root *, sector_t); -- 2.39.5