[PATCHSET 0/4] struct request optimizations

[PATCHSET 0/4] struct request optimizations

[Jens Axboe]

With the latest patchset from Tejun, we grew the request structure
a little bit. It's been quite a while since I've taken a look at
the layout of the structure, this patchset is a first attempt at
doing that.

One advantage of Tejun's patchset is that we no longer rely on
the atomic complete flag on blk-mq. We can use that to shuffle
some bits, and reclaim the full atomic_flags field.

Cache optimize the layout of struct request a bit, to group
things a little more logically. Not a huge shuffle, just a few
select members.

We end up doing better in synthetic testing after this, details
in the last patch.

-- 
Jens Axboe

[PATCH 1/4] block: remove REQ_ATOM_POLL_SLEPT

[Jens Axboe]

We don't need this to be an atomic flag, it can be a regular
flag. We either end up on the same CPU for the polling, in which
case the state is sane, or we did the sleep which would imply
the needed barrier to ensure we see the right state.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-mq-debugfs.c | 1 -
 block/blk-mq.c         | 5 ++---
 block/blk.h            | 2 --
 include/linux/blkdev.h | 2 ++
 4 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 8adc83786256..2a9c9f8b6162 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -294,7 +294,6 @@ static const char *const rqf_name[] = {
 #define RQAF_NAME(name) [REQ_ATOM_##name] = #name
 static const char *const rqaf_name[] = {
 	RQAF_NAME(COMPLETE),
-	RQAF_NAME(POLL_SLEPT),
 };
 #undef RQAF_NAME
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9aa24c9508f9..faa31814983c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -483,7 +483,6 @@ void blk_mq_free_request(struct request *rq)
 		blk_put_rl(blk_rq_rl(rq));
 
 	blk_mq_rq_update_state(rq, MQ_RQ_IDLE);
-	clear_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
 	if (rq->tag != -1)
 		blk_mq_put_tag(hctx, hctx->tags, ctx, rq->tag);
 	if (sched_tag != -1)
@@ -2970,7 +2969,7 @@ static bool blk_mq_poll_hybrid_sleep(struct request_queue *q,
 	unsigned int nsecs;
 	ktime_t kt;
 
-	if (test_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags))
+	if (rq->rq_flags & RQF_MQ_POLL_SLEPT)
 		return false;
 
 	/*
@@ -2990,7 +2989,7 @@ static bool blk_mq_poll_hybrid_sleep(struct request_queue *q,
 	if (!nsecs)
 		return false;
 
-	set_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
+	rq->rq_flags |= RQF_MQ_POLL_SLEPT;
 
 	/*
 	 * This will be replaced with the stats tracking code, using
diff --git a/block/blk.h b/block/blk.h
index a68dbe312ea3..eb306c52121e 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -124,8 +124,6 @@ void blk_account_io_done(struct request *req);
  */
 enum rq_atomic_flags {
 	REQ_ATOM_COMPLETE = 0,
-
-	REQ_ATOM_POLL_SLEPT,
 };
 
 /*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 007a7cf1f262..ba31674d8581 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -127,6 +127,8 @@ typedef __u32 __bitwise req_flags_t;
 #define RQF_ZONE_WRITE_LOCKED	((__force req_flags_t)(1 << 19))
 /* timeout is expired */
 #define RQF_MQ_TIMEOUT_EXPIRED	((__force req_flags_t)(1 << 20))
+/* already slept for hybrid poll */
+#define RQF_MQ_POLL_SLEPT	((__force req_flags_t)(1 << 21))
 
 /* flags that prevent us from merging requests: */
 #define RQF_NOMERGE_FLAGS \
-- 
2.7.4

[PATCH 2/4] block: add accessors for setting/querying request deadline

[Jens Axboe]

We reduce the resolution of request expiry, but since we're already
using jiffies for this where resolution depends on the kernel
configuration and since the timeout resolution is coarse anyway,
that should be fine.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-mq.c         |  2 +-
 block/blk-timeout.c    | 14 ++++++++------
 block/blk.h            | 13 +++++++++++++
 include/linux/blkdev.h |  4 +++-
 4 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index faa31814983c..d875c51bcff8 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -858,7 +858,7 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
 	while (true) {
 		start = read_seqcount_begin(&rq->gstate_seq);
 		gstate = READ_ONCE(rq->gstate);
-		deadline = rq->deadline;
+		deadline = blk_rq_deadline(rq);
 		if (!read_seqcount_retry(&rq->gstate_seq, start))
 			break;
 		cond_resched();
diff --git a/block/blk-timeout.c b/block/blk-timeout.c
index ebe99963386c..a05e3676d24a 100644
--- a/block/blk-timeout.c
+++ b/block/blk-timeout.c
@@ -112,7 +112,9 @@ static void blk_rq_timed_out(struct request *req)
 static void blk_rq_check_expired(struct request *rq, unsigned long *next_timeout,
 			  unsigned int *next_set)
 {
-	if (time_after_eq(jiffies, rq->deadline)) {
+	const unsigned long deadline = blk_rq_deadline(rq);
+
+	if (time_after_eq(jiffies, deadline)) {
 		list_del_init(&rq->timeout_list);
 
 		/*
@@ -120,8 +122,8 @@ static void blk_rq_check_expired(struct request *rq, unsigned long *next_timeout
 		 */
 		if (!blk_mark_rq_complete(rq))
 			blk_rq_timed_out(rq);
-	} else if (!*next_set || time_after(*next_timeout, rq->deadline)) {
-		*next_timeout = rq->deadline;
+	} else if (!*next_set || time_after(*next_timeout, deadline)) {
+		*next_timeout = deadline;
 		*next_set = 1;
 	}
 }
@@ -162,7 +164,7 @@ void blk_abort_request(struct request *req)
 		 * immediately and that scan sees the new timeout value.
 		 * No need for fancy synchronizations.
 		 */
-		req->deadline = jiffies;
+		blk_rq_set_deadline(req, jiffies);
 		mod_timer(&req->q->timeout, 0);
 	} else {
 		if (blk_mark_rq_complete(req))
@@ -213,7 +215,7 @@ void blk_add_timer(struct request *req)
 	if (!req->timeout)
 		req->timeout = q->rq_timeout;
 
-	req->deadline = jiffies + req->timeout;
+	blk_rq_set_deadline(req, jiffies + req->timeout);
 	req->rq_flags &= ~RQF_MQ_TIMEOUT_EXPIRED;
 
 	/*
@@ -228,7 +230,7 @@ void blk_add_timer(struct request *req)
 	 * than an existing one, modify the timer. Round up to next nearest
 	 * second.
 	 */
-	expiry = blk_rq_timeout(round_jiffies_up(req->deadline));
+	expiry = blk_rq_timeout(round_jiffies_up(blk_rq_deadline(req)));
 
 	if (!timer_pending(&q->timeout) ||
 	    time_before(expiry, q->timeout.expires)) {
diff --git a/block/blk.h b/block/blk.h
index eb306c52121e..8b26a8872e05 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -237,6 +237,19 @@ static inline void req_set_nomerge(struct request_queue *q, struct request *req)
 }
 
 /*
+ * Steal a bit from this field for legacy IO path atomic IO marking
+ */
+static inline void blk_rq_set_deadline(struct request *rq, unsigned long time)
+{
+	rq->__deadline = time & ~0x1;
+}
+
+static inline unsigned long blk_rq_deadline(struct request *rq)
+{
+	return rq->__deadline & ~0x1;
+}
+
+/*
  * Internal io_context interface
  */
 void get_io_context(struct io_context *ioc);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index ba31674d8581..aa6698cf483c 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -257,7 +257,9 @@ struct request {
 	struct u64_stats_sync aborted_gstate_sync;
 	u64 aborted_gstate;
 
-	unsigned long deadline;
+	/* access through blk_rq_set_deadline, blk_rq_deadline */
+	unsigned long __deadline;
+
 	struct list_head timeout_list;
 
 	/*
-- 
2.7.4

Re: [PATCH 2/4] block: add accessors for setting/querying request deadline

[Bart Van Assche]

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byB0aGUgUkhTIGNvbnN0YW50Pw0KDQpUaGFua3MsDQoNCkJhcnQu

Re: [PATCH 2/4] block: add accessors for setting/querying request deadline

[Jens Axboe]

On 1/9/18 11:40 AM, Bart Van Assche wrote:
> On Tue, 2018-01-09 at 11:27 -0700, Jens Axboe wrote:
>> +static inline void blk_rq_set_deadline(struct request *rq, unsigned long time)
>> +{
>> +	rq->__deadline = time & ~0x1;
>> +}
>> +
>> +static inline unsigned long blk_rq_deadline(struct request *rq)
>> +{
>> +	return rq->__deadline & ~0x1;
>> +}
> 
> Hello Jens,
> 
> The type of rq->__deadline is "unsigned long" but the type of the right-hand
> side constant is int. Shouldn't an "UL" suffix be added to the RHS constant?

Good catch, yeah you're right. I'll make that change, thanks.

-- 
Jens Axboe

[PATCH 3/4] block: convert REQ_ATOM_COMPLETE to stealing rq->__deadline bit

[Jens Axboe]

We only have one atomic flag left. Instead of using an entire
unsigned long for that, steal the bottom bit of the deadline
field that we already reserved.

Remove ->atomic_flags, since it's now unused.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-core.c       |  2 +-
 block/blk-mq-debugfs.c |  8 --------
 block/blk.h            | 19 +++++++++----------
 include/linux/blkdev.h |  2 --
 4 files changed, 10 insertions(+), 21 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index f843ae4f858d..7ba607527487 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2853,7 +2853,7 @@ void blk_start_request(struct request *req)
 		wbt_issue(req->q->rq_wb, &req->issue_stat);
 	}
 
-	BUG_ON(test_bit(REQ_ATOM_COMPLETE, &req->atomic_flags));
+	BUG_ON(blk_rq_is_complete(req));
 	blk_add_timer(req);
 }
 EXPORT_SYMBOL(blk_start_request);
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 2a9c9f8b6162..ac99b78415ec 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -291,12 +291,6 @@ static const char *const rqf_name[] = {
 };
 #undef RQF_NAME
 
-#define RQAF_NAME(name) [REQ_ATOM_##name] = #name
-static const char *const rqaf_name[] = {
-	RQAF_NAME(COMPLETE),
-};
-#undef RQAF_NAME
-
 int __blk_mq_debugfs_rq_show(struct seq_file *m, struct request *rq)
 {
 	const struct blk_mq_ops *const mq_ops = rq->q->mq_ops;
@@ -313,8 +307,6 @@ int __blk_mq_debugfs_rq_show(struct seq_file *m, struct request *rq)
 	seq_puts(m, ", .rq_flags=");
 	blk_flags_show(m, (__force unsigned int)rq->rq_flags, rqf_name,
 		       ARRAY_SIZE(rqf_name));
-	seq_puts(m, ", .atomic_flags=");
-	blk_flags_show(m, rq->atomic_flags, rqaf_name, ARRAY_SIZE(rqaf_name));
 	seq_printf(m, ", .tag=%d, .internal_tag=%d", rq->tag,
 		   rq->internal_tag);
 	if (mq_ops->show_rq)
diff --git a/block/blk.h b/block/blk.h
index 8b26a8872e05..d5ae07cc4abb 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -120,24 +120,23 @@ void blk_account_io_completion(struct request *req, unsigned int bytes);
 void blk_account_io_done(struct request *req);
 
 /*
- * Internal atomic flags for request handling
- */
-enum rq_atomic_flags {
-	REQ_ATOM_COMPLETE = 0,
-};
-
-/*
  * EH timer and IO completion will both attempt to 'grab' the request, make
- * sure that only one of them succeeds
+ * sure that only one of them succeeds. Steal the bottom bit of the
+ * __deadline field for this.
  */
 static inline int blk_mark_rq_complete(struct request *rq)
 {
-	return test_and_set_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
+	return test_and_set_bit(0, &rq->__deadline);
 }
 
 static inline void blk_clear_rq_complete(struct request *rq)
 {
-	clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
+	clear_bit(0, &rq->__deadline);
+}
+
+static inline bool blk_rq_is_complete(struct request *rq)
+{
+	return test_bit(0, &rq->__deadline);
 }
 
 /*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index aa6698cf483c..d4b2f7bb18d6 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -156,8 +156,6 @@ struct request {
 
 	int internal_tag;
 
-	unsigned long atomic_flags;
-
 	/* the following two fields are internal, NEVER access directly */
 	unsigned int __data_len;	/* total data len */
 	int tag;
-- 
2.7.4

Re: [PATCH 3/4] block: convert REQ_ATOM_COMPLETE to stealing rq->__deadline bit

[Bart Van Assche]

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YW5rcywNCg0KQmFydC4=

Re: [PATCH 3/4] block: convert REQ_ATOM_COMPLETE to stealing rq->__deadline bit

[Jens Axboe]

On 1/9/18 11:43 AM, Bart Van Assche wrote:
> On Tue, 2018-01-09 at 11:27 -0700, Jens Axboe wrote:
>>  static inline int blk_mark_rq_complete(struct request *rq)
>>  {
>> -	return test_and_set_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
>> +	return test_and_set_bit(0, &rq->__deadline);
>>  }
>>  
>>  static inline void blk_clear_rq_complete(struct request *rq)
>>  {
>> -	clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
>> +	clear_bit(0, &rq->__deadline);
>> +}
>> +
>> +static inline bool blk_rq_is_complete(struct request *rq)
>> +{
>> +	return test_bit(0, &rq->__deadline);
>>  }
> 
> Hello Jens,
> 
> With this change setting or changing the deadline clears the COMPLETE flag.
> Is that the intended behavior? If so, should perhaps a comment be added above
> blk_rq_set_deadline()?

Yeah, it's intentional. I can add a comment to that effect. It's only done
before queueing - except for the case where we force a timeout, but for that
it's only on the blk-mq side, which doesn't care.

-- 
Jens Axboe

Re: [PATCH 3/4] block: convert REQ_ATOM_COMPLETE to stealing rq->__deadline bit

[Jens Axboe]

On 1/9/18 11:44 AM, Jens Axboe wrote:
> On 1/9/18 11:43 AM, Bart Van Assche wrote:
>> On Tue, 2018-01-09 at 11:27 -0700, Jens Axboe wrote:
>>>  static inline int blk_mark_rq_complete(struct request *rq)
>>>  {
>>> -	return test_and_set_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
>>> +	return test_and_set_bit(0, &rq->__deadline);
>>>  }
>>>  
>>>  static inline void blk_clear_rq_complete(struct request *rq)
>>>  {
>>> -	clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
>>> +	clear_bit(0, &rq->__deadline);
>>> +}
>>> +
>>> +static inline bool blk_rq_is_complete(struct request *rq)
>>> +{
>>> +	return test_bit(0, &rq->__deadline);
>>>  }
>>
>> Hello Jens,
>>
>> With this change setting or changing the deadline clears the COMPLETE flag.
>> Is that the intended behavior? If so, should perhaps a comment be added above
>> blk_rq_set_deadline()?
> 
> Yeah, it's intentional. I can add a comment to that effect. It's only done
> before queueing - except for the case where we force a timeout, but for that
> it's only on the blk-mq side, which doesn't care.

Since we clear it when we init the request, we could also just leave the
bit intact when setting the deadline. That's probably the safer choice:

static inline void blk_rq_set_deadline(struct request *rq, unsigned long time)  
{                                                                               
        rq->__deadline = (time & ~0x1UL) | (rq->__deadline & 0x1UL);
}

I'll test that, previous testing didn't find anything wrong with clearing
the bit, but this does seem safer.

-- 
Jens Axboe

[PATCH 4/4] block: rearrange a few request fields for better cache layout

[Jens Axboe]

Move completion related items (like the call single data) near the
end of the struct, instead of mixing them in with the initial
queueing related fields.

Move queuelist below the bio structures. Then we have all
queueing related bits in the first cache line.

This yields a 1.5-2% increase in IOPS for a null_blk test, both for
sync and for high thread count access. Sync test goes form 975K to
992K, 32-thread case from 20.8M to 21.2M IOPS.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/blkdev.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index d4b2f7bb18d6..1b2472f6662e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -141,12 +141,6 @@ typedef __u32 __bitwise req_flags_t;
  * especially blk_mq_rq_ctx_init() to take care of the added fields.
  */
 struct request {
-	struct list_head queuelist;
-	union {
-		call_single_data_t csd;
-		u64 fifo_time;
-	};
-
 	struct request_queue *q;
 	struct blk_mq_ctx *mq_ctx;
 
@@ -164,6 +158,8 @@ struct request {
 	struct bio *bio;
 	struct bio *biotail;
 
+	struct list_head queuelist;
+
 	/*
 	 * The hash is used inside the scheduler, and killed once the
 	 * request reaches the dispatch list. The ipi_list is only used
@@ -260,6 +256,11 @@ struct request {
 
 	struct list_head timeout_list;
 
+	union {
+		call_single_data_t csd;
+		u64 fifo_time;
+	};
+
 	/*
 	 * completion callback.
 	 */
-- 
2.7.4