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Date: Mon, 24 Oct 2016 06:27:11 -0800
From: Kent Overstreet <kent.overstreet@...il.com>
To: Davidlohr Bueso <dave@...olabs.net>,
Eric Wheeler <git@...ux.ewheeler.net>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-bcache@...r.kernel.org
Subject: Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:
Restructure wait loop)
On Sun, Oct 23, 2016 at 06:57:26PM -0700, Davidlohr Bueso wrote:
> On Wed, 19 Oct 2016, Peter Zijlstra wrote:
>
> > Subject: sched: Better explain sleep/wakeup
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Wed Oct 19 15:45:27 CEST 2016
> >
> > There were a few questions wrt how sleep-wakeup works. Try and explain
> > it more.
> >
> > Requested-by: Will Deacon <will.deacon@....com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > include/linux/sched.h | 52 ++++++++++++++++++++++++++++++++++----------------
> > kernel/sched/core.c | 15 +++++++-------
> > 2 files changed, 44 insertions(+), 23 deletions(-)
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -262,20 +262,9 @@ extern char ___assert_task_state[1 - 2*!
> > #define set_task_state(tsk, state_value) \
> > do { \
> > (tsk)->task_state_change = _THIS_IP_; \
> > - smp_store_mb((tsk)->state, (state_value)); \
> > + smp_store_mb((tsk)->state, (state_value)); \
> > } while (0)
> >
> > -/*
> > - * set_current_state() includes a barrier so that the write of current->state
> > - * is correctly serialised wrt the caller's subsequent test of whether to
> > - * actually sleep:
> > - *
> > - * set_current_state(TASK_UNINTERRUPTIBLE);
> > - * if (do_i_need_to_sleep())
> > - * schedule();
> > - *
> > - * If the caller does not need such serialisation then use __set_current_state()
> > - */
> > #define __set_current_state(state_value) \
> > do { \
> > current->task_state_change = _THIS_IP_; \
> > @@ -284,11 +273,19 @@ extern char ___assert_task_state[1 - 2*!
> > #define set_current_state(state_value) \
> > do { \
> > current->task_state_change = _THIS_IP_; \
> > - smp_store_mb(current->state, (state_value)); \
> > + smp_store_mb(current->state, (state_value)); \
> > } while (0)
> >
> > #else
> >
> > +/*
> > + * @tsk had better be current, or you get to keep the pieces.
>
> That reminds me we were getting rid of the set_task_state() calls. Bcache was
> pending, being only user in the kernel that doesn't actually use current; but
> instead breaks newly (yet blocked/uninterruptible) created garbage collection
> kthread. I cannot figure out why this is done (ie purposely accounting the
> load avg. Furthermore gc kicks in in very specific scenarios obviously, such
> as as by the allocator task, so I don't see why bcache gc should want to be
> interruptible.
>
> Kent, Jens, can we get rid of this?
Here's a patch that just fixes the way gc gets woken up. Eric, you want to take
this?
-- >8 --
Subject: [PATCH] bcache: Make gc wakeup saner
This lets us ditch a set_task_state() call.
Signed-off-by: Kent Overstreet <kent.overstreet@...il.com>
---
drivers/md/bcache/bcache.h | 4 ++--
drivers/md/bcache/btree.c | 39 ++++++++++++++++++++-------------------
drivers/md/bcache/btree.h | 3 +--
drivers/md/bcache/request.c | 4 +---
drivers/md/bcache/super.c | 2 ++
5 files changed, 26 insertions(+), 26 deletions(-)
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 6b420a55c7..c3ea03c9a1 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -425,7 +425,7 @@ struct cache {
* until a gc finishes - otherwise we could pointlessly burn a ton of
* cpu
*/
- unsigned invalidate_needs_gc:1;
+ unsigned invalidate_needs_gc;
bool discard; /* Get rid of? */
@@ -593,8 +593,8 @@ struct cache_set {
/* Counts how many sectors bio_insert has added to the cache */
atomic_t sectors_to_gc;
+ wait_queue_head_t gc_wait;
- wait_queue_head_t moving_gc_wait;
struct keybuf moving_gc_keys;
/* Number of moving GC bios in flight */
struct semaphore moving_in_flight;
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 81d3db40cd..2efdce0724 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1757,32 +1757,34 @@ static void bch_btree_gc(struct cache_set *c)
bch_moving_gc(c);
}
-static int bch_gc_thread(void *arg)
+static bool gc_should_run(struct cache_set *c)
{
- struct cache_set *c = arg;
struct cache *ca;
unsigned i;
- while (1) {
-again:
- bch_btree_gc(c);
+ for_each_cache(ca, c, i)
+ if (ca->invalidate_needs_gc)
+ return true;
- set_current_state(TASK_INTERRUPTIBLE);
- if (kthread_should_stop())
- break;
+ if (atomic_read(&c->sectors_to_gc) < 0)
+ return true;
- mutex_lock(&c->bucket_lock);
+ return false;
+}
- for_each_cache(ca, c, i)
- if (ca->invalidate_needs_gc) {
- mutex_unlock(&c->bucket_lock);
- set_current_state(TASK_RUNNING);
- goto again;
- }
+static int bch_gc_thread(void *arg)
+{
+ struct cache_set *c = arg;
- mutex_unlock(&c->bucket_lock);
+ while (1) {
+ wait_event_interruptible(c->gc_wait,
+ kthread_should_stop() || gc_should_run(c));
- schedule();
+ if (kthread_should_stop())
+ break;
+
+ set_gc_sectors(c);
+ bch_btree_gc(c);
}
return 0;
@@ -1790,11 +1792,10 @@ static int bch_gc_thread(void *arg)
int bch_gc_thread_start(struct cache_set *c)
{
- c->gc_thread = kthread_create(bch_gc_thread, c, "bcache_gc");
+ c->gc_thread = kthread_run(bch_gc_thread, c, "bcache_gc");
if (IS_ERR(c->gc_thread))
return PTR_ERR(c->gc_thread);
- set_task_state(c->gc_thread, TASK_INTERRUPTIBLE);
return 0;
}
diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
index 5c391fa01b..9b80417cd5 100644
--- a/drivers/md/bcache/btree.h
+++ b/drivers/md/bcache/btree.h
@@ -260,8 +260,7 @@ void bch_initial_mark_key(struct cache_set *, int, struct bkey *);
static inline void wake_up_gc(struct cache_set *c)
{
- if (c->gc_thread)
- wake_up_process(c->gc_thread);
+ wake_up(&c->gc_wait);
}
#define MAP_DONE 0
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 40ffe5e424..a37c1776f2 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -196,10 +196,8 @@ static void bch_data_insert_start(struct closure *cl)
struct data_insert_op *op = container_of(cl, struct data_insert_op, cl);
struct bio *bio = op->bio, *n;
- if (atomic_sub_return(bio_sectors(bio), &op->c->sectors_to_gc) < 0) {
- set_gc_sectors(op->c);
+ if (atomic_sub_return(bio_sectors(bio), &op->c->sectors_to_gc) < 0)
wake_up_gc(op->c);
- }
if (op->bypass)
return bch_data_invalidate(cl);
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 849ad441cd..66669c8f41 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1491,6 +1491,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
mutex_init(&c->bucket_lock);
init_waitqueue_head(&c->btree_cache_wait);
init_waitqueue_head(&c->bucket_wait);
+ init_waitqueue_head(&c->gc_wait);
sema_init(&c->uuid_write_mutex, 1);
spin_lock_init(&c->btree_gc_time.lock);
@@ -1550,6 +1551,7 @@ static void run_cache_set(struct cache_set *c)
for_each_cache(ca, c, i)
c->nbuckets += ca->sb.nbuckets;
+ set_gc_sectors(c);
if (CACHE_SYNC(&c->sb)) {
LIST_HEAD(journal);
--
2.9.3
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