svn commit: r344903 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs

[Alexander Motin]

Author: mav
Date: Thu Mar  7 22:56:39 2019
New Revision: 344903
URL: https://svnweb.freebsd.org/changeset/base/344903

Log:
  Improve entropy for ZFS taskqueue selection.
  
  I just found that at least on Skylake CPUs cpu_ticks() never returns odd
  values, only even, and possibly has even bigger step (176/2?), that makes
  its lower bits very bad entropy source, leaving half of taskqueues unused.
  Switch to sbinuptime(), closer to upstreams, mitigates the problem by the
  rate conversion working as kind of hash function.  In case that is somehow
  not enough (timer rate is too low or too divisible) mix in curcpu.
  
  MFC after:	1 week

Modified:
  head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c

Modified: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c
==============================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c	Thu Mar  7 22:51:58 2019	(r344902)
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c	Thu Mar  7 22:56:39 2019	(r344903)
@@ -1070,7 +1070,8 @@ spa_taskq_dispatch_ent(spa_t *spa, zio_type_t t, zio_t
 		tq = tqs->stqs_taskq[0];
 	} else {
 #ifdef _KERNEL
-		tq = tqs->stqs_taskq[cpu_ticks() % tqs->stqs_count];
+		tq = tqs->stqs_taskq[(u_int)(sbinuptime() + curcpu) %
+		    tqs->stqs_count];
 #else
 		tq = tqs->stqs_taskq[gethrtime() % tqs->stqs_count];
 #endif