Re: [PATCH v2] dm crypt: add flags to optionally bypass dm-crypt workqueues

[Damien Le Moal]

On 2020/06/27 6:03, Ignat Korchagin wrote:
> This is a follow up from [1]. Consider the following script:
> 
> sudo modprobe brd rd_nr=1 rd_size=4194304
> 
> echo '0 8388608 crypt capi:ecb(cipher_null) - 0 /dev/ram0 0' | \
> sudo dmsetup create eram0
> 
> echo '0 8388608 crypt capi:ecb(cipher_null) - 0 /dev/ram0 0 1 no_write_workqueue' | \
> sudo dmsetup create eram0-inline-write
> 
> echo '0 8388608 crypt capi:ecb(cipher_null) - 0 /dev/ram0 0 1 no_read_workqueue' | \
> sudo dmsetup create eram0-inline-read
> 
> devices="/dev/ram0 /dev/mapper/eram0 /dev/mapper/eram0-inline-read "
> devices+="/dev/mapper/eram0-inline-write"
> 
> for dev in $devices; do
>   echo "reading from $dev"
>   sudo fio --filename=$dev --readwrite=read --bs=4k --direct=1 \
>   --loops=1000000 --runtime=3m --name=plain | grep READ
> done
> 
> for dev in $devices; do
>   echo "writing to $dev"
>   sudo fio --filename=$dev --readwrite=write --bs=4k --direct=1 \
>   --loops=1000000 --runtime=3m --name=plain | grep WRITE
> done
> 
> This script creates a ramdisk (to eliminate hardware bias in the benchmark) and
> three dm-crypt instances on top. All dm-crypt instances use the NULL cipher
> to eliminate potentially expensive crypto bias (the NULL cipher just uses memcpy
> for "encyption"). The first instance is the current dm-crypt implementation from
> 5.8-rc2, the two others have new optional flags enabled, which bypass kcryptd
> workqueues for reads and writes respectively and write sorting for writes. On
> my VM (Debian in VirtualBox with 4 cores on 2.8 GHz Quad-Core Intel Core i7) I
> get the following output (formatted for better readability):
> 
> reading from /dev/ram0
>    READ: bw=508MiB/s (533MB/s), 508MiB/s-508MiB/s (533MB/s-533MB/s), io=89.3GiB (95.9GB), run=180000-180000msec
> 
> reading from /dev/mapper/eram0
>    READ: bw=80.6MiB/s (84.5MB/s), 80.6MiB/s-80.6MiB/s (84.5MB/s-84.5MB/s), io=14.2GiB (15.2GB), run=180000-180000msec
> 
> reading from /dev/mapper/eram0-inline-read
>    READ: bw=295MiB/s (309MB/s), 295MiB/s-295MiB/s (309MB/s-309MB/s), io=51.8GiB (55.6GB), run=180000-180000msec
> 
> reading from /dev/mapper/eram0-inline-write
>    READ: bw=114MiB/s (120MB/s), 114MiB/s-114MiB/s (120MB/s-120MB/s), io=20.1GiB (21.5GB), run=180000-180000msec
> 
> writing to /dev/ram0
>   WRITE: bw=516MiB/s (541MB/s), 516MiB/s-516MiB/s (541MB/s-541MB/s), io=90.7GiB (97.4GB), run=180001-180001msec
> 
> writing to /dev/mapper/eram0
>   WRITE: bw=40.4MiB/s (42.4MB/s), 40.4MiB/s-40.4MiB/s (42.4MB/s-42.4MB/s), io=7271MiB (7624MB), run=180001-180001msec
> 
> writing to /dev/mapper/eram0-inline-read
>   WRITE: bw=38.9MiB/s (40.8MB/s), 38.9MiB/s-38.9MiB/s (40.8MB/s-40.8MB/s), io=7000MiB (7340MB), run=180001-180001msec
> 
> writing to /dev/mapper/eram0-inline-write
>   WRITE: bw=277MiB/s (290MB/s), 277MiB/s-277MiB/s (290MB/s-290MB/s), io=48.6GiB (52.2GB), run=180000-180000msec
> 
> Current dm-crypt implementation creates a significant IO performance overhead
> (at least on small IO block sizes) for both latency and throughput. We suspect
> offloading IO request processing into workqueues and async threads is more
> harmful these days with the modern fast storage. I also did some digging into
> the dm-crypt git history and much of this async processing is not needed
> anymore, because the reasons it was added are mostly gone from the kernel. More
> details can be found in [2] (see "Git archeology" section).
> 
> This change adds no_(read|write)_workqueue flags separately for read and write
> BIOs, which direct dm-crypt not to offload crypto operations into kcryptd
> workqueues and process everything inline. In addition, writes are not buffered
> to be sorted in the dm-crypt red-black tree, but dispatched immediately. For
> cases, where crypto operations cannot happen inline (hard interrupt context,
> for example the read path of some NVME drivers), we offload the work to a
> tasklet rather than a workqueue.
> 
> These flags ensure inline BIO processing in the dm-crypt module only. It is
> worth noting that some Crypto API implementations may offload encryption into
> their own workqueues, which are independent of the dm-crypt and its
> configuration. However upon enabling no_(read|write)_workqueue flags dm-crypt
> will instruct Crypto API not to backlog crypto requests.
> 
> [1]: https://www.spinics.net/lists/dm-crypt/msg07516.html
> [2]: https://blog.cloudflare.com/speeding-up-linux-disk-encryption/
> 
> Signed-off-by: Ignat Korchagin <ignat@xxxxxxxxxxxxxx>
> ---
>  drivers/md/dm-crypt.c | 68 +++++++++++++++++++++++++++++++++----------
>  1 file changed, 52 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index 000ddfab5ba0..6924eb49b1df 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -69,6 +69,7 @@ struct dm_crypt_io {
>  	u8 *integrity_metadata;
>  	bool integrity_metadata_from_pool;
>  	struct work_struct work;
> +	struct tasklet_struct tasklet;
>  
>  	struct convert_context ctx;
>  
> @@ -127,7 +128,8 @@ struct iv_elephant_private {
>   * and encrypts / decrypts at the same time.
>   */
>  enum flags { DM_CRYPT_SUSPENDED, DM_CRYPT_KEY_VALID,
> -	     DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD };
> +	     DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD,
> +	     DM_CRYPT_NO_READ_WORKQUEUE, DM_CRYPT_NO_WRITE_WORKQUEUE };

I liked the "INLINE" naming. What about DM_CRYPT_READ_INLINE and
DM_CRYPT_WRITE_INLINE ? Shorter too :)

But from the changes below, it looks like your change is now less about being
purely inline or synchronous but about bypassing the workqueue.
Is this correct ?

>  
>  enum cipher_flags {
>  	CRYPT_MODE_INTEGRITY_AEAD,	/* Use authenticated mode for cihper */
> @@ -1449,7 +1451,7 @@ static void kcryptd_async_done(struct crypto_async_request *async_req,
>  			       int error);
>  
>  static void crypt_alloc_req_skcipher(struct crypt_config *cc,
> -				     struct convert_context *ctx)
> +				     struct convert_context *ctx, bool nobacklog)
>  {
>  	unsigned key_index = ctx->cc_sector & (cc->tfms_count - 1);
>  
> @@ -1463,12 +1465,12 @@ static void crypt_alloc_req_skcipher(struct crypt_config *cc,
>  	 * requests if driver request queue is full.
>  	 */
>  	skcipher_request_set_callback(ctx->r.req,
> -	    CRYPTO_TFM_REQ_MAY_BACKLOG,
> +	    nobacklog ? 0 : CRYPTO_TFM_REQ_MAY_BACKLOG,
>  	    kcryptd_async_done, dmreq_of_req(cc, ctx->r.req));

Will not specifying CRYPTO_TFM_REQ_MAY_BACKLOG always cause the crypto API to
return -EBUSY ? From the comment above the skcipher_request_set_callback(), it
seems that this will be the case only if the skcipher diver queue is full. So in
other word, keeping the kcryptd_async_done() callback and executing the skcipher
request through crypt_convert() and crypt_convert_block_skcipher() may still end
up being an asynchronous operation. Can you confirm this and is it what you
intended to implement ?

>From my understanding of the crypto API, and from what Eric commented, a truly
synchronous/inline execution of the skcypher needs a call like:

crypto_wait_req(crypto_skcipher_encrypt(req), &wait);

For SMR use case were we must absolutely preserve the write requests order, the
above change will probably be needed. Will check again.

>  }
>  
>  static void crypt_alloc_req_aead(struct crypt_config *cc,
> -				 struct convert_context *ctx)
> +				 struct convert_context *ctx, bool nobacklog)
>  {
>  	if (!ctx->r.req_aead)
>  		ctx->r.req_aead = mempool_alloc(&cc->req_pool, GFP_NOIO);
> @@ -1480,17 +1482,17 @@ static void crypt_alloc_req_aead(struct crypt_config *cc,
>  	 * requests if driver request queue is full.
>  	 */
>  	aead_request_set_callback(ctx->r.req_aead,
> -	    CRYPTO_TFM_REQ_MAY_BACKLOG,
> +	    nobacklog ? 0 : CRYPTO_TFM_REQ_MAY_BACKLOG,
>  	    kcryptd_async_done, dmreq_of_req(cc, ctx->r.req_aead));
>  }
>  
>  static void crypt_alloc_req(struct crypt_config *cc,
> -			    struct convert_context *ctx)
> +			    struct convert_context *ctx, bool nobacklog)
>  {
>  	if (crypt_integrity_aead(cc))
> -		crypt_alloc_req_aead(cc, ctx);
> +		crypt_alloc_req_aead(cc, ctx, nobacklog);
>  	else
> -		crypt_alloc_req_skcipher(cc, ctx);
> +		crypt_alloc_req_skcipher(cc, ctx, nobacklog);
>  }
>  
>  static void crypt_free_req_skcipher(struct crypt_config *cc,
> @@ -1523,7 +1525,7 @@ static void crypt_free_req(struct crypt_config *cc, void *req, struct bio *base_
>   * Encrypt / decrypt data from one bio to another one (can be the same one)
>   */
>  static blk_status_t crypt_convert(struct crypt_config *cc,
> -			 struct convert_context *ctx)
> +			 struct convert_context *ctx, bool noresched)

"noresched" is named after what will happen, not after the reason for it. So to
clarify, why not rename this as "convert_inline" or "do_inline" ?

>  {
>  	unsigned int tag_offset = 0;
>  	unsigned int sector_step = cc->sector_size >> SECTOR_SHIFT;
> @@ -1533,7 +1535,7 @@ static blk_status_t crypt_convert(struct crypt_config *cc,
>  
>  	while (ctx->iter_in.bi_size && ctx->iter_out.bi_size) {
>  
> -		crypt_alloc_req(cc, ctx);
> +		crypt_alloc_req(cc, ctx, noresched);
>  		atomic_inc(&ctx->cc_pending);
>  
>  		if (crypt_integrity_aead(cc))
> @@ -1566,7 +1568,8 @@ static blk_status_t crypt_convert(struct crypt_config *cc,
>  			atomic_dec(&ctx->cc_pending);
>  			ctx->cc_sector += sector_step;
>  			tag_offset++;
> -			cond_resched();
> +			if (!noresched)
> +				cond_resched();
>  			continue;
>  		/*
>  		 * There was a data integrity error.
> @@ -1879,6 +1882,9 @@ static void kcryptd_crypt_write_io_submit(struct dm_crypt_io *io, int async)
>  	unsigned long flags;
>  	sector_t sector;
>  	struct rb_node **rbp, *parent;
> +	bool nosort =
> +			(likely(!async) && test_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags)) ||
> +			test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags);

"nosort" is a little obscure as a name. Why not just "do_inline" ? In any case,
since this bool is used only in the if below, you could just write the condition
directly there.

>  
>  	if (unlikely(io->error)) {
>  		crypt_free_buffer_pages(cc, clone);
> @@ -1892,7 +1898,7 @@ static void kcryptd_crypt_write_io_submit(struct dm_crypt_io *io, int async)
>  
>  	clone->bi_iter.bi_sector = cc->start + io->sector;
>  
> -	if (likely(!async) && test_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags)) {
> +	if (nosort) {
>  		generic_make_request(clone);
>  		return;
>  	}
> @@ -1941,7 +1947,7 @@ static void kcryptd_crypt_write_convert(struct dm_crypt_io *io)
>  	sector += bio_sectors(clone);
>  
>  	crypt_inc_pending(io);
> -	r = crypt_convert(cc, &io->ctx);
> +	r = crypt_convert(cc, &io->ctx, test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags));
>  	if (r)
>  		io->error = r;
>  	crypt_finished = atomic_dec_and_test(&io->ctx.cc_pending);
> @@ -1971,7 +1977,7 @@ static void kcryptd_crypt_read_convert(struct dm_crypt_io *io)
>  	crypt_convert_init(cc, &io->ctx, io->base_bio, io->base_bio,
>  			   io->sector);
>  
> -	r = crypt_convert(cc, &io->ctx);
> +	r = crypt_convert(cc, &io->ctx, test_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags));
>  	if (r)
>  		io->error = r;
>  
> @@ -2031,9 +2037,29 @@ static void kcryptd_crypt(struct work_struct *work)
>  		kcryptd_crypt_write_convert(io);
>  }
>  
> +static void kcryptd_crypt_tasklet(unsigned long work)
> +{
> +	kcryptd_crypt((struct work_struct *)work);
> +}
> +
>  static void kcryptd_queue_crypt(struct dm_crypt_io *io)
>  {
>  	struct crypt_config *cc = io->cc;
> +	bool noworkqueue =
> +			(bio_data_dir(io->base_bio) == READ && test_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags)) ||
> +			(bio_data_dir(io->base_bio) == WRITE && test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags));

Since this variable is used only in the if statement bleow, why not used the
condition directly in that statement ?

> +
> +	if (noworkqueue) {
> +		if (in_irq()) {
> +			/* Crypto API's "skcipher_walk_first() refuses to work in hard IRQ context */
> +			tasklet_init(&io->tasklet, kcryptd_crypt_tasklet, (unsigned long)&io->work);
> +			tasklet_schedule(&io->tasklet);
> +			return;
> +		}
> +
> +		kcryptd_crypt(&io->work);
> +		return;
> +	}
>  
>  	INIT_WORK(&io->work, kcryptd_crypt);
>  	queue_work(cc->crypt_queue, &io->work);
> @@ -2838,7 +2864,7 @@ static int crypt_ctr_optional(struct dm_target *ti, unsigned int argc, char **ar
>  	struct crypt_config *cc = ti->private;
>  	struct dm_arg_set as;
>  	static const struct dm_arg _args[] = {
> -		{0, 6, "Invalid number of feature args"},
> +		{0, 8, "Invalid number of feature args"},
>  	};
>  	unsigned int opt_params, val;
>  	const char *opt_string, *sval;
> @@ -2868,6 +2894,10 @@ static int crypt_ctr_optional(struct dm_target *ti, unsigned int argc, char **ar
>  
>  		else if (!strcasecmp(opt_string, "submit_from_crypt_cpus"))
>  			set_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags);
> +		else if (!strcasecmp(opt_string, "no_read_workqueue"))
> +			set_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags);
> +		else if (!strcasecmp(opt_string, "no_write_workqueue"))
> +			set_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags);
>  		else if (sscanf(opt_string, "integrity:%u:", &val) == 1) {
>  			if (val == 0 || val > MAX_TAG_SIZE) {
>  				ti->error = "Invalid integrity arguments";
> @@ -3196,6 +3226,8 @@ static void crypt_status(struct dm_target *ti, status_type_t type,
>  		num_feature_args += !!ti->num_discard_bios;
>  		num_feature_args += test_bit(DM_CRYPT_SAME_CPU, &cc->flags);
>  		num_feature_args += test_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags);
> +		num_feature_args += test_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags);
> +		num_feature_args += test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags);
>  		num_feature_args += cc->sector_size != (1 << SECTOR_SHIFT);
>  		num_feature_args += test_bit(CRYPT_IV_LARGE_SECTORS, &cc->cipher_flags);
>  		if (cc->on_disk_tag_size)
> @@ -3208,6 +3240,10 @@ static void crypt_status(struct dm_target *ti, status_type_t type,
>  				DMEMIT(" same_cpu_crypt");
>  			if (test_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags))
>  				DMEMIT(" submit_from_crypt_cpus");
> +			if (test_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags))
> +				DMEMIT(" no_read_workqueue");
> +			if (test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags))
> +				DMEMIT(" no_write_workqueue");
>  			if (cc->on_disk_tag_size)
>  				DMEMIT(" integrity:%u:%s", cc->on_disk_tag_size, cc->cipher_auth);
>  			if (cc->sector_size != (1 << SECTOR_SHIFT))
> @@ -3320,7 +3356,7 @@ static void crypt_io_hints(struct dm_target *ti, struct queue_limits *limits)
>  
>  static struct target_type crypt_target = {
>  	.name   = "crypt",
> -	.version = {1, 21, 0},
> +	.version = {1, 22, 0},
>  	.module = THIS_MODULE,
>  	.ctr    = crypt_ctr,
>  	.dtr    = crypt_dtr,
> 


-- 
Damien Le Moal
Western Digital Research