Don't scale zfs_zevent_len_max by CPU count

The lower bound for this scaling to too low and the upper bound is too
high.  Use a fixed default length of 512 instead, which is a reasonable
value on any system.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11822

man/man5/zfs-module-parameters.5

@@ -14,7 +14,7 @@
 .\" CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
-.TH ZFS-MODULE-PARAMETERS 5 "Aug 24, 2020" OpenZFS
+.TH ZFS-MODULE-PARAMETERS 5 "Mar 31, 2021" OpenZFS
 .SH NAME
 zfs\-module\-parameters \- ZFS module parameters
 .SH DESCRIPTION
@@ -3845,11 +3845,10 @@ Use \fB1\fR for yes and \fB0\fR for no (default).
 \fBzfs_zevent_len_max\fR (int)
 .ad
 .RS 12n
-Max event queue length. A value of 0 will result in a calculated value which
-increases with the number of CPUs in the system (minimum 64 events). Events
-in the queue can be viewed with the \fBzpool events\fR command.
+Max event queue length.
+Events in the queue can be viewed with the \fBzpool events\fR command.
 .sp
-Default value: \fB0\fR.
+Default value: \fB512\fR.
 .RE
 
 .sp

module/zfs/fm.c

@@ -69,7 +69,7 @@
 #include <sys/console.h>
 #include <sys/zfs_ioctl.h>
 
-int zfs_zevent_len_max = 0;
+int zfs_zevent_len_max = 512;
 int zfs_zevent_cols = 80;
 int zfs_zevent_console = 0;
 
@@ -1620,9 +1620,6 @@ fm_init(void)
 	zevent_len_cur = 0;
 	zevent_flags = 0;
 
-	if (zfs_zevent_len_max == 0)
-		zfs_zevent_len_max = ERPT_MAX_ERRS * MAX(max_ncpus, 4);
-
 	/* Initialize zevent allocation and generation kstats */
 	fm_ksp = kstat_create("zfs", 0, "fm", "misc", KSTAT_TYPE_NAMED,
 	    sizeof (struct erpt_kstat) / sizeof (kstat_named_t),