Message-ID: <c1b45434-6341-4e26-a376-4f434df2caf0@vivo.com>
Date: Sat, 11 May 2024 19:08:39 +0800
From: YangYang <yang.yang@...o.com>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>,
 dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] dm: Avoid sending redundant empty flush bios to the
 same block device

On 2024/5/10 22:08, Mikulas Patocka wrote:
> Hi
> 
> Regarding *bitmap = bitmap_zalloc(t->num_devices + 1, GFP_KERNEL); - you
> can't allocate memory in the I/O processing path, because it may deadlock.
> Think of a case when the system is out of memory and it needs to swap out
> some pages and the swap out operation attempts to allocate more memory.

Got it!

> 
> Anyway, the patch is too complicated. I suggest to try this:
> 
> * introduce a per-target bit "flush_pass_around" that is set for dm-linear
>    and dm-stripe and that means that the target supports flush optimization
> 
> * set a per-table "flush_pass_around" bit if all the targets in the table
>    have "flush_pass_around" set
> 
> * in __send_empty_flush, test the table's "flush_pass_around" bit and if
>    it is set, bypass this loop "for (unsigned int i = 0; i <
>    t->num_targets; i++) {" and iterate over dm_table->devices and send the
>    flush to each of them.

OK, I will do this in the next version.

Thanks for your comments.

> 
> Hopefully, these changes will make the patch smaller and more acceptable.
> 
> Mikulas
> 
> 
> 
> On Mon, 22 Apr 2024, Yang Yang wrote:
> 
>> __send_empty_flush() sends empty flush bios to every target in the
>> dm_table. However, if the num_targets exceeds the number of block
>> devices in the dm_table's device list, it could lead to multiple
>> invocations of __send_duplicate_bios() for the same block device.
>> Typically, a single thread sending numerous empty flush bios to one
>> block device is redundant, as these bios are likely to be merged by the
>> flush state machine. In scenarios where num_targets significantly
>> outweighs the number of block devices, such behavior may result in a
>> noteworthy decrease in performance.
>>
>> This issue can be reproduced using this command line:
>>    for i in {0..1023}; do
>>      echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
>>    done | dmsetup create example
>>
>> With this fix, a random write with fsync workload executed with the
>> following fio command:
>>
>>    fio --group_reporting --name=benchmark --filename=/dev/mapper/example \
>>        --ioengine=sync --invalidate=1 --numjobs=16 --rw=randwrite \
>>        --blocksize=4k --size=2G --time_based --runtime=30 --fdatasync=1
>>
>> results in an increase from 857 KB/s to 30.8 MB/s of the write
>> throughput (3580% increase).
>>
>> Signed-off-by: Yang Yang <yang.yang@...o.com>
>> ---
>>   drivers/md/dm-core.h          |  1 +
>>   drivers/md/dm-table.c         |  7 +++++
>>   drivers/md/dm.c               | 59 +++++++++++++++++++++++++++++++++++
>>   include/linux/device-mapper.h |  6 ++++
>>   4 files changed, 73 insertions(+)
>>
>> diff --git a/drivers/md/dm-core.h b/drivers/md/dm-core.h
>> index e6757a30dcca..7e3f2168289f 100644
>> --- a/drivers/md/dm-core.h
>> +++ b/drivers/md/dm-core.h
>> @@ -217,6 +217,7 @@ struct dm_table {
>>   	/* a list of devices used by this table */
>>   	struct list_head devices;
>>   	struct rw_semaphore devices_lock;
>> +	unsigned short num_devices;
>>   
>>   	/* events get handed up using this callback */
>>   	void (*event_fn)(void *data);
>> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
>> index 41f1d731ae5a..ddc60e498afb 100644
>> --- a/drivers/md/dm-table.c
>> +++ b/drivers/md/dm-table.c
>> @@ -2133,6 +2133,8 @@ void dm_table_postsuspend_targets(struct dm_table *t)
>>   
>>   int dm_table_resume_targets(struct dm_table *t)
>>   {
>> +	struct list_head *devices = dm_table_get_devices(t);
>> +	struct dm_dev_internal *dd;
>>   	unsigned int i;
>>   	int r = 0;
>>   
>> @@ -2159,6 +2161,11 @@ int dm_table_resume_targets(struct dm_table *t)
>>   			ti->type->resume(ti);
>>   	}
>>   
>> +	t->num_devices = 0;
>> +
>> +	list_for_each_entry(dd, devices, list)
>> +		dd->dm_dev->index = ++(t->num_devices);
>> +
>>   	return 0;
>>   }
>>   
>> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>> index 56aa2a8b9d71..7297235291f6 100644
>> --- a/drivers/md/dm.c
>> +++ b/drivers/md/dm.c
>> @@ -48,6 +48,8 @@
>>    */
>>   #define REQ_DM_POLL_LIST	REQ_DRV
>>   
>> +#define DM_MAX_TABLE_DEVICES	1024
>> +
>>   static const char *_name = DM_NAME;
>>   
>>   static unsigned int major;
>> @@ -1543,10 +1545,38 @@ static unsigned int __send_duplicate_bios(struct clone_info *ci, struct dm_targe
>>   	return ret;
>>   }
>>   
>> +static inline bool has_redundant_flush(struct dm_table *t,
>> +		unsigned long **bitmap)
>> +{
>> +	if (t->num_devices < t->num_targets) {
>> +		/* Add a limit here to prevent excessive memory usage for bitmaps */
>> +		if (t->num_devices >= DM_MAX_TABLE_DEVICES)
>> +			return false;
>> +
>> +		/* dm_dev's index starts from 1, so need plus 1 here */
>> +		*bitmap = bitmap_zalloc(t->num_devices + 1, GFP_KERNEL);
>> +		if (*bitmap)
>> +			return true;
>> +	}
>> +
>> +	return false;
>> +}
>> +
>> +static int dm_get_dev_index(struct dm_target *ti, struct dm_dev *dev,
>> +				     sector_t start, sector_t len, void *data)
>> +{
>> +	unsigned short *index = data;
>> +	*index = dev->index;
>> +	return 0;
>> +}
>> +
>>   static void __send_empty_flush(struct clone_info *ci)
>>   {
>>   	struct dm_table *t = ci->map;
>>   	struct bio flush_bio;
>> +	unsigned long *handled_map;
>> +	unsigned int nr_handled = 0;
>> +	bool check = has_redundant_flush(t, &handled_map);
>>   
>>   	/*
>>   	 * Use an on-stack bio for this, it's safe since we don't
>> @@ -1562,17 +1592,46 @@ static void __send_empty_flush(struct clone_info *ci)
>>   
>>   	for (unsigned int i = 0; i < t->num_targets; i++) {
>>   		unsigned int bios;
>> +		unsigned short index = 0;
>>   		struct dm_target *ti = dm_table_get_target(t, i);
>>   
>>   		if (unlikely(ti->num_flush_bios == 0))
>>   			continue;
>>   
>> +		/*
>> +		 * If the num_targets is greater than the number of block devices
>> +		 * in the dm_table's devices list, __send_empty_flush() might
>> +		 * invoke __send_duplicate_bios() multiple times for the same
>> +		 * block device. This could lead to a substantial decrease in
>> +		 * performance when num_targets significantly exceeds the number
>> +		 * of block devices.
>> +		 * Ensure that __send_duplicate_bios() is only called once for
>> +		 * each block device.
>> +		 */
>> +		if (check) {
>> +			if (nr_handled == t->num_devices)
>> +				break;
>> +
>> +			if (ti->type->iterate_devices)
>> +				ti->type->iterate_devices(ti, dm_get_dev_index, &index);
>> +
>> +			if (index > 0) {
>> +				if (__test_and_set_bit(index, handled_map))
>> +					continue;
>> +				else
>> +					nr_handled++;
>> +			}
>> +		}
>> +
>>   		atomic_add(ti->num_flush_bios, &ci->io->io_count);
>>   		bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
>>   					     NULL, GFP_NOWAIT);
>>   		atomic_sub(ti->num_flush_bios - bios, &ci->io->io_count);
>>   	}
>>   
>> +	if (check)
>> +		bitmap_free(handled_map);
>> +
>>   	/*
>>   	 * alloc_io() takes one extra reference for submission, so the
>>   	 * reference won't reach 0 without the following subtraction
>> diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
>> index 82b2195efaca..4a54b4f0a609 100644
>> --- a/include/linux/device-mapper.h
>> +++ b/include/linux/device-mapper.h
>> @@ -169,6 +169,12 @@ struct dm_dev {
>>   	struct dax_device *dax_dev;
>>   	blk_mode_t mode;
>>   	char name[16];
>> +
>> +	/*
>> +	 * sequential number for each dm_dev in dm_table's devices list,
>> +	 * start from 1
>> +	 */
>> +	unsigned short index;
>>   };
>>   
>>   /*
>> -- 
>> 2.34.1
>>
> 
> 

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