| Message ID | 20250507-ublk_task_per_io-v6-2-a2a298783c01@purestorage.com |
|---|---|
| State | New |
| Headers | show |
| Series | ublk: decouple server threads from hctxs | expand |
On Wed, May 07, 2025 at 03:49:36PM -0600, Uday Shankar wrote: > In update_alloc_hint_after_get, we wrap the new hint back to 0 one bit > too early. This breaks round robin tag allocation (BLK_MQ_F_TAG_RR) - > some tags get skipped, so we don't get round robin tags even in the > simple case of single-threaded load on a single hctx. Fix the off-by-one > in the wrapping condition so that round robin tag allocation works > properly. > > The same pattern occurs in __sbitmap_get_word, so fix it there too. > > Signed-off-by: Uday Shankar <ushankar@purestorage.com> > --- > lib/sbitmap.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/lib/sbitmap.c b/lib/sbitmap.c > index d3412984170c03dc6600bbe53f130404b765ac5a..aa1cec78b9649f1f3e8ef2d617dd7ee724391a8c 100644 > --- a/lib/sbitmap.c > +++ b/lib/sbitmap.c > @@ -51,7 +51,7 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb, > } else if (nr == hint || unlikely(sb->round_robin)) { > /* Only update the hint if we used it. */ > hint = nr + 1; > - if (hint >= depth - 1) > + if (hint >= depth) > hint = 0; > this_cpu_write(*sb->alloc_hint, hint); This may help for round robin. > } > @@ -182,7 +182,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth, > break; > > hint = nr + 1; > - if (hint >= depth - 1) > + if (hint >= depth) > hint = 0; I guess round robin may need to return -1 if 'hint >= depth'. Also the existing behavior starts from adding sbitmap, maybe Jens has idea why hint is checked against 'depth -1' instead of 'depth'. Thanks, Ming
On Wed, May 7, 2025 at 2:49 PM Uday Shankar <ushankar@purestorage.com> wrote: > > In update_alloc_hint_after_get, we wrap the new hint back to 0 one bit > too early. This breaks round robin tag allocation (BLK_MQ_F_TAG_RR) - > some tags get skipped, so we don't get round robin tags even in the > simple case of single-threaded load on a single hctx. Fix the off-by-one > in the wrapping condition so that round robin tag allocation works > properly. > > The same pattern occurs in __sbitmap_get_word, so fix it there too. Should this have a Fixes tag? Looks like the off-by-one wrapping has existed since 4bb659b15699 ("blk-mq: implement new and more efficient tagging scheme"), but it's only a correctness issue with round-robin tag allocation, which was added in 24391c0dc57c ("blk-mq: add tag allocation policy"). I don't have much background on blk-mq's round-robin tag allocation, but FWIW, Reviewed-by: Caleb Sander Mateos <csander@purestorage.com> > > Signed-off-by: Uday Shankar <ushankar@purestorage.com> > --- > lib/sbitmap.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/lib/sbitmap.c b/lib/sbitmap.c > index d3412984170c03dc6600bbe53f130404b765ac5a..aa1cec78b9649f1f3e8ef2d617dd7ee724391a8c 100644 > --- a/lib/sbitmap.c > +++ b/lib/sbitmap.c > @@ -51,7 +51,7 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb, > } else if (nr == hint || unlikely(sb->round_robin)) { > /* Only update the hint if we used it. */ > hint = nr + 1; > - if (hint >= depth - 1) > + if (hint >= depth) > hint = 0; > this_cpu_write(*sb->alloc_hint, hint); > } > @@ -182,7 +182,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth, > break; > > hint = nr + 1; > - if (hint >= depth - 1) > + if (hint >= depth) > hint = 0; > } > > > -- > 2.34.1 >
diff --git a/lib/sbitmap.c b/lib/sbitmap.c index d3412984170c03dc6600bbe53f130404b765ac5a..aa1cec78b9649f1f3e8ef2d617dd7ee724391a8c 100644 --- a/lib/sbitmap.c +++ b/lib/sbitmap.c @@ -51,7 +51,7 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb, } else if (nr == hint || unlikely(sb->round_robin)) { /* Only update the hint if we used it. */ hint = nr + 1; - if (hint >= depth - 1) + if (hint >= depth) hint = 0; this_cpu_write(*sb->alloc_hint, hint); } @@ -182,7 +182,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth, break; hint = nr + 1; - if (hint >= depth - 1) + if (hint >= depth) hint = 0; }
In update_alloc_hint_after_get, we wrap the new hint back to 0 one bit too early. This breaks round robin tag allocation (BLK_MQ_F_TAG_RR) - some tags get skipped, so we don't get round robin tags even in the simple case of single-threaded load on a single hctx. Fix the off-by-one in the wrapping condition so that round robin tag allocation works properly. The same pattern occurs in __sbitmap_get_word, so fix it there too. Signed-off-by: Uday Shankar <ushankar@purestorage.com> --- lib/sbitmap.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)