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

From: Ignat Korchagin
Date: Tue Jun 30 2020 - 05:35:05 EST

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On Tue, Jun 30, 2020 at 3:51 AM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
>
> 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 ?

Yes, from the test with the NULL cipher it is clearly visible that
workqueues are the main cause of the performance degradation. The
previous patch actually did the same thing with the addition of a
custom xts-proxy synchronous module, which achieved "full inline"
processing. But it is clear now, that inline/non-inline Crypto API
does not change much from a performance point of view.

> >
> > 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 ?

Yes, so far these flags should bypass dm-crypt workqueues only. I had
a quick look around CRYPTO_TFM_REQ_MAY_BACKLOG and it seems that both
generic xts as well as aesni implementations (and other crypto
involved in disk encryption) do not have any logic related to the
flag, so we may as well leave it as is.

> 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.

I think this is not an "inline" execution, rather blocking the current
thread and waiting for the potential asynchronous crypto thread to
finish its operation.
It seems we have different use-cases here. By bypassing workqueues we
just want to improve performance, but otherwise do not really care
about the order of requests.
Waiting for crypto to complete synchronously may actually decrease
performance, but required to preserve the order in some cases. Should
this be a yet another flag?

> > }
> >
> > 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

Next message: Christophe Kerello: "Re: [PATCH v5 4/6] memory: stm32-fmc2-ebi: add STM32 FMC2 EBI controller driver"
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In reply to: Damien Le Moal: "Re: [PATCH v2] dm crypt: add flags to optionally bypass dm-crypt workqueues"
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