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Subject: Re: [PATCH -next RFC] block: support to account io_ticks precisely
To: Yu Kuai <yukuai1@huaweicloud.com>, bvanassche@acm.org, hch@lst.de,
 ming.lei@redhat.com, axboe@kernel.dk
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
 yi.zhang@huawei.com, yangerkun@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20231205093743.1823351-1-yukuai1@huaweicloud.com>
From: Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <e1475e94-2026-c154-8e4d-498aca4f7ed8@huaweicloud.com>
Date: Tue, 2 Jan 2024 18:49:57 +0800
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friendly ping...

There used be lots of attempt to implement similiar feature, please let
me know if this feature is still interested from mainline. The
additional overhead can be ignored at least for HHD, and we had received
multiple complaint that disk util becomes worse after upgrading kernel.
We do provide additional tools like blktrace and bpftrace script to get
precise util, but for performance and userspace complexity, most of our
custormers prefer to keep using iostat of read /proc/diskstats.

Thanks,
Kuai

?? 2023/12/05 17:37, Yu Kuai д??:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Currently, io_ticks is accounted based on sampling, specifically
> update_io_ticks() will always account io_ticks by 1 jiffies from
> bdev_start_io_acct()/blk_account_io_start(), and the result can be
> inaccurate, for example(HZ is 250):
> 
> Test script:
> fio -filename=/dev/sda -bs=4k -rw=write -direct=1 -name=test -thinktime=4ms
> 
> Test result: util is about 90%, while the disk is really idle.
> 
> In order to account io_ticks precisely, update_io_ticks() must know if
> there are IO inflight already, and this requires overhead slightly,
> hence precise io accounting is disabled by default, and user can enable
> it through sysfs entry.
> 
> Noted that for rq-based devcie, part_stat_local_inc/dec() and
> part_in_flight() is used to track inflight instead of iterating tags,
> which is not supposed to be used in fast path because 'tags->lock' is
> grabbed in blk_mq_find_and_get_req().
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>   Documentation/ABI/stable/sysfs-block |  8 ++++--
>   block/blk-core.c                     | 17 ++++++++----
>   block/blk-mq.c                       | 18 ++++++++++---
>   block/blk-sysfs.c                    | 40 ++++++++++++++++++++++++++--
>   block/blk.h                          |  4 ++-
>   block/genhd.c                        |  6 ++---
>   include/linux/blk-mq.h               |  1 +
>   include/linux/blkdev.h               |  3 +++
>   8 files changed, 80 insertions(+), 17 deletions(-)
> 
> diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
> index 1fe9a553c37b..e5fedecf7bdf 100644
> --- a/Documentation/ABI/stable/sysfs-block
> +++ b/Documentation/ABI/stable/sysfs-block
> @@ -358,8 +358,12 @@ What:		/sys/block/<disk>/queue/iostats
>   Date:		January 2009
>   Contact:	linux-block@vger.kernel.org
>   Description:
> -		[RW] This file is used to control (on/off) the iostats
> -		accounting of the disk.
> +		[RW] This file is used to control the iostats accounting of the
> +		disk. If this value is 0, iostats accounting is disabled; If
> +		this value is 1, iostats accounting is enabled, but io_ticks is
> +		accounted by sampling and the result is not accurate; If this
> +		value is 2, iostats accounting is enabled and io_ticks is
> +		accounted precisely, but there will be slightly overhead.
>   
>   
>   What:		/sys/block/<disk>/queue/logical_block_size
> diff --git a/block/blk-core.c b/block/blk-core.c
> index fdf25b8d6e78..405883d606cd 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -935,14 +935,20 @@ int iocb_bio_iopoll(struct kiocb *kiocb, struct io_comp_batch *iob,
>   }
>   EXPORT_SYMBOL_GPL(iocb_bio_iopoll);
>   
> -void update_io_ticks(struct block_device *part, unsigned long now, bool end)
> +void update_io_ticks(struct block_device *part, unsigned long now, bool end,
> +		     bool precise)
>   {
>   	unsigned long stamp;
>   again:
>   	stamp = READ_ONCE(part->bd_stamp);
> -	if (unlikely(time_after(now, stamp))) {
> -		if (likely(try_cmpxchg(&part->bd_stamp, &stamp, now)))
> +	if (unlikely(time_after(now, stamp)) &&
> +	    likely(try_cmpxchg(&part->bd_stamp, &stamp, now))) {
> +		if (precise) {
> +			if (end || part_in_flight(part))
> +				__part_stat_add(part, io_ticks, now - stamp);
> +		} else {
>   			__part_stat_add(part, io_ticks, end ? now - stamp : 1);
> +		}
>   	}
>   	if (part->bd_partno) {
>   		part = bdev_whole(part);
> @@ -954,7 +960,8 @@ unsigned long bdev_start_io_acct(struct block_device *bdev, enum req_op op,
>   				 unsigned long start_time)
>   {
>   	part_stat_lock();
> -	update_io_ticks(bdev, start_time, false);
> +	update_io_ticks(bdev, start_time, false,
> +			blk_queue_precise_io_stat(bdev->bd_queue));
>   	part_stat_local_inc(bdev, in_flight[op_is_write(op)]);
>   	part_stat_unlock();
>   
> @@ -982,7 +989,7 @@ void bdev_end_io_acct(struct block_device *bdev, enum req_op op,
>   	unsigned long duration = now - start_time;
>   
>   	part_stat_lock();
> -	update_io_ticks(bdev, now, true);
> +	update_io_ticks(bdev, now, true, true);
>   	part_stat_inc(bdev, ios[sgrp]);
>   	part_stat_add(bdev, sectors[sgrp], sectors);
>   	part_stat_add(bdev, nsecs[sgrp], jiffies_to_nsecs(duration));
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 900c1be1fee1..84fffbdc9dc1 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -360,8 +360,11 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>   
>   	if (data->flags & BLK_MQ_REQ_PM)
>   		data->rq_flags |= RQF_PM;
> -	if (blk_queue_io_stat(q))
> +	if (blk_queue_io_stat(q)) {
>   		data->rq_flags |= RQF_IO_STAT;
> +		if (blk_queue_precise_io_stat(q))
> +			data->rq_flags |= RQF_PRECISE_IO_STAT;
> +	}
>   	rq->rq_flags = data->rq_flags;
>   
>   	if (data->rq_flags & RQF_SCHED_TAGS) {
> @@ -996,17 +999,21 @@ static inline void blk_account_io_done(struct request *req, u64 now)
>   		const int sgrp = op_stat_group(req_op(req));
>   
>   		part_stat_lock();
> -		update_io_ticks(req->part, jiffies, true);
> +		update_io_ticks(req->part, jiffies, true, true);
>   		part_stat_inc(req->part, ios[sgrp]);
>   		part_stat_add(req->part, nsecs[sgrp], now - req->start_time_ns);
> +		if (req->rq_flags & RQF_PRECISE_IO_STAT)
> +			part_stat_local_dec(req->part,
> +					in_flight[op_is_write(req_op(req))]);
>   		part_stat_unlock();
>   	}
>   }
>   
>   static inline void blk_account_io_start(struct request *req)
>   {
> -	trace_block_io_start(req);
> +	bool precise = req->rq_flags & RQF_PRECISE_IO_STAT;
>   
> +	trace_block_io_start(req);
>   	if (blk_do_io_stat(req)) {
>   		/*
>   		 * All non-passthrough requests are created from a bio with one
> @@ -1020,7 +1027,10 @@ static inline void blk_account_io_start(struct request *req)
>   			req->part = req->q->disk->part0;
>   
>   		part_stat_lock();
> -		update_io_ticks(req->part, jiffies, false);
> +		update_io_ticks(req->part, jiffies, false, precise);
> +		if (precise)
> +			part_stat_local_inc(req->part,
> +					in_flight[op_is_write(req_op(req))]);
>   		part_stat_unlock();
>   	}
>   }
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 63e481262336..75ddbbd1edb4 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -303,7 +303,6 @@ queue_##name##_store(struct request_queue *q, const char *page, size_t count) \
>   
>   QUEUE_SYSFS_BIT_FNS(nonrot, NONROT, 1);
>   QUEUE_SYSFS_BIT_FNS(random, ADD_RANDOM, 0);
> -QUEUE_SYSFS_BIT_FNS(iostats, IO_STAT, 0);
>   QUEUE_SYSFS_BIT_FNS(stable_writes, STABLE_WRITES, 0);
>   #undef QUEUE_SYSFS_BIT_FNS
>   
> @@ -473,6 +472,43 @@ static ssize_t queue_dax_show(struct request_queue *q, char *page)
>   	return queue_var_show(blk_queue_dax(q), page);
>   }
>   
> +static ssize_t queue_iostats_show(struct request_queue *q, char *page)
> +{
> +	int val = 0;
> +
> +	if (blk_queue_io_stat(q))
> +		val = blk_queue_precise_io_stat(q) ? 2 : 1;
> +
> +	return sprintf(page, "%u\n", val);
> +}
> +
> +static ssize_t
> +queue_iostats_store(struct request_queue *q, const char *page, size_t count)
> +{
> +	unsigned long nr;
> +	int ret, err;
> +
> +	ret = queue_var_store(&nr, page, count);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (nr > 2)
> +		return -EINVAL;
> +
> +	if (nr == 0) {
> +		blk_queue_flag_clear(QUEUE_FLAG_IO_STAT, q);
> +		blk_queue_flag_clear(QUEUE_FLAG_PRECISE_IO_STAT, q);
> +	} else if (nr == 1) {
> +		blk_queue_flag_set(QUEUE_FLAG_IO_STAT, q);
> +		blk_queue_flag_clear(QUEUE_FLAG_PRECISE_IO_STAT, q);
> +	} else {
> +		blk_queue_flag_set(QUEUE_FLAG_IO_STAT, q);
> +		blk_queue_flag_set(QUEUE_FLAG_PRECISE_IO_STAT, q);
> +	}
> +
> +	return count;
> +}
> +
>   #define QUEUE_RO_ENTRY(_prefix, _name)			\
>   static struct queue_sysfs_entry _prefix##_entry = {	\
>   	.attr	= { .name = _name, .mode = 0444 },	\
> @@ -494,6 +530,7 @@ QUEUE_RO_ENTRY(queue_max_segments, "max_segments");
>   QUEUE_RO_ENTRY(queue_max_integrity_segments, "max_integrity_segments");
>   QUEUE_RO_ENTRY(queue_max_segment_size, "max_segment_size");
>   QUEUE_RW_ENTRY(elv_iosched, "scheduler");
> +QUEUE_RW_ENTRY(queue_iostats, "iostats");
>   
>   QUEUE_RO_ENTRY(queue_logical_block_size, "logical_block_size");
>   QUEUE_RO_ENTRY(queue_physical_block_size, "physical_block_size");
> @@ -539,7 +576,6 @@ static struct queue_sysfs_entry queue_hw_sector_size_entry = {
>   };
>   
>   QUEUE_RW_ENTRY(queue_nonrot, "rotational");
> -QUEUE_RW_ENTRY(queue_iostats, "iostats");
>   QUEUE_RW_ENTRY(queue_random, "add_random");
>   QUEUE_RW_ENTRY(queue_stable_writes, "stable_writes");
>   
> diff --git a/block/blk.h b/block/blk.h
> index 08a358bc0919..d2321709a9ce 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -343,7 +343,9 @@ static inline bool blk_do_io_stat(struct request *rq)
>   	return (rq->rq_flags & RQF_IO_STAT) && !blk_rq_is_passthrough(rq);
>   }
>   
> -void update_io_ticks(struct block_device *part, unsigned long now, bool end);
> +void update_io_ticks(struct block_device *part, unsigned long now, bool end,
> +		     bool precise);
> +unsigned int part_in_flight(struct block_device *part);
>   
>   static inline void req_set_nomerge(struct request_queue *q, struct request *req)
>   {
> diff --git a/block/genhd.c b/block/genhd.c
> index c9d06f72c587..aec651976c83 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -118,7 +118,7 @@ static void part_stat_read_all(struct block_device *part,
>   	}
>   }
>   
> -static unsigned int part_in_flight(struct block_device *part)
> +unsigned int part_in_flight(struct block_device *part)
>   {
>   	unsigned int inflight = 0;
>   	int cpu;
> @@ -962,7 +962,7 @@ ssize_t part_stat_show(struct device *dev,
>   
>   	if (inflight) {
>   		part_stat_lock();
> -		update_io_ticks(bdev, jiffies, true);
> +		update_io_ticks(bdev, jiffies, true, true);
>   		part_stat_unlock();
>   	}
>   	part_stat_read_all(bdev, &stat);
> @@ -1255,7 +1255,7 @@ static int diskstats_show(struct seq_file *seqf, void *v)
>   
>   		if (inflight) {
>   			part_stat_lock();
> -			update_io_ticks(hd, jiffies, true);
> +			update_io_ticks(hd, jiffies, true, true);
>   			part_stat_unlock();
>   		}
>   		part_stat_read_all(hd, &stat);
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 1ab3081c82ed..e6c7adea1b4e 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -44,6 +44,7 @@ typedef __u32 __bitwise req_flags_t;
>   #define RQF_QUIET		((__force req_flags_t)(1 << 11))
>   /* account into disk and partition IO statistics */
>   #define RQF_IO_STAT		((__force req_flags_t)(1 << 13))
> +#define RQF_PRECISE_IO_STAT	((__force req_flags_t)(1 << 14))
>   /* runtime pm request */
>   #define RQF_PM			((__force req_flags_t)(1 << 15))
>   /* on IO scheduler merge hash */
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 51fa7ffdee83..fbc7ac419288 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -534,6 +534,7 @@ struct request_queue {
>   #define QUEUE_FLAG_NONROT	6	/* non-rotational device (SSD) */
>   #define QUEUE_FLAG_VIRT		QUEUE_FLAG_NONROT /* paravirt device */
>   #define QUEUE_FLAG_IO_STAT	7	/* do disk/partitions IO accounting */
> +#define QUEUE_FLAG_PRECISE_IO_STAT 8	/* do disk/partitions IO accounting precisely */
>   #define QUEUE_FLAG_NOXMERGES	9	/* No extended merges */
>   #define QUEUE_FLAG_ADD_RANDOM	10	/* Contributes to random pool */
>   #define QUEUE_FLAG_SYNCHRONOUS	11	/* always completes in submit context */
> @@ -574,6 +575,8 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);
>   #define blk_queue_stable_writes(q) \
>   	test_bit(QUEUE_FLAG_STABLE_WRITES, &(q)->queue_flags)
>   #define blk_queue_io_stat(q)	test_bit(QUEUE_FLAG_IO_STAT, &(q)->queue_flags)
> +#define blk_queue_precise_io_stat(q) \
> +	test_bit(QUEUE_FLAG_PRECISE_IO_STAT, &(q)->queue_flags)
>   #define blk_queue_add_random(q)	test_bit(QUEUE_FLAG_ADD_RANDOM, &(q)->queue_flags)
>   #define blk_queue_zone_resetall(q)	\
>   	test_bit(QUEUE_FLAG_ZONE_RESETALL, &(q)->queue_flags)
>