dm verity: stop using WQ_UNBOUND for verify_wq

Setting WQ_UNBOUND increases scheduler latency on ARM64.  This is
likely due to the asymmetric architecture of ARM64 processors.

I've been unable to reproduce the results that claim WQ_UNBOUND gives
a performance boost on x86-64.

This flag is causing performance issues for multiple subsystems within
Android.  Notably, the same slowdown exists for decompression with
EROFS.

| open-prebuilt-camera  | WQ_UNBOUND | ~WQ_UNBOUND   |
|-----------------------|------------|---------------|
| verity wait time (us) | 11746      | 119 (-98%)    |
| erofs wait time (us)  | 357805     | 174205 (-51%) |

| sha256 ramdisk random read | WQ_UNBOUND    | ~WQ_UNBOUND |
|----------------------------|-----------=---|-------------|
| arm64 (accelerated)        | bw=42.4MiB/s  | bw=212MiB/s |
| arm64 (generic)            | bw=16.5MiB/s  | bw=48MiB/s  |
| x86_64 (generic)           | bw=233MiB/s   | bw=230MiB/s |

Using a alloc_workqueue() @max_active arg of num_online_cpus() only
made sense with WQ_UNBOUND. Switch the @max_active arg to 0 (aka
default, which is 256 per-cpu).

Also, eliminate 'wq_flags' since it really doesn't serve a purpose.

Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index ccf5b852fbf7a741c019cfe689d5675282f17265..4c52b056ecf08a5c8413cba648a0e9f6e8248aa9 100644 (file)
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -1162,7 +1162,6 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
        struct dm_verity_sig_opts verify_args = {0};
        struct dm_arg_set as;
        unsigned int num;
-       unsigned int wq_flags;
        unsigned long long num_ll;
        int r;
        int i;
@@ -1399,8 +1398,6 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
                goto bad;
        }
 
-       /* WQ_UNBOUND greatly improves performance when running on ramdisk */
-       wq_flags = WQ_MEM_RECLAIM | WQ_UNBOUND;
        /*
         * Using WQ_HIGHPRI improves throughput and completion latency by
         * reducing wait times when reading from a dm-verity device.
@@ -1410,8 +1407,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
         * will fall-back to using it for error handling (or if the bufio cache
         * doesn't have required hashes).
         */
-       wq_flags |= WQ_HIGHPRI;
-       v->verify_wq = alloc_workqueue("kverityd", wq_flags, num_online_cpus());
+       v->verify_wq = alloc_workqueue("kverityd", WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
        if (!v->verify_wq) {
                ti->error = "Cannot allocate workqueue";
                r = -ENOMEM;