ZEN: INTERACTIVE: dm-crypt: Disable workqueues for crypto ops

Queueing in dm-crypt for crypto operations reduces performance on modern
systems.  As discussed in an article from Cloudflare, they discovered
that queuing was introduced because the crypto subsystem used to be
synchronous.  Since it's now asynchronous, we get double queueing when
using the subsystem through dm-crypt.  This is obviously undesirable and
reduces throughput and increases latency.

Disable queueing when using our Zen Interactive configuration.

Fixes: zen-kernel#282
Signed-off-by: Kai Krakow <kai@kaishome.de>

Author: damentz

drivers/md/dm-crypt.c

@@ -3308,6 +3308,11 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 			goto bad;
 	}
 
+#ifdef CONFIG_ZEN_INTERACTIVE
+	set_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags);
+	set_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags);
+#endif
+
 	ret = crypt_ctr_cipher(ti, argv[0], argv[1]);
 	if (ret < 0)
 		goto bad;

init/Kconfig

@@ -172,6 +172,7 @@ config ZEN_INTERACTIVE
 
 	    Default scheduler for SQ..: mq-deadline ->   bfq
 	    Default scheduler for MQ..:        none ->   kyber
+	    DM-Crypt workqueues.......:        yes  ->   no
 
 	  --- Virtual Memory Subsystem ---------------------------