On 7/05/2022 10:04 am, Alan Somers wrote:
The branch main has been updated by asomers:
URL:
https://cgit.FreeBSD.org/src/commit/?id=1d2421ad8b6d508ef155752bdfc5948f7373bac3
commit 1d2421ad8b6d508ef155752bdfc5948f7373bac3
Author: Alan Somers <asom...@freebsd.org>
AuthorDate: 2022-05-05 21:35:23 +0000
Commit: Alan Somers <asom...@freebsd.org>
CommitDate: 2022-05-06 23:25:43 +0000
Correctly measure system load averages > 1024
The old fixed-point arithmetic used for calculating load averages had an
overflow at 1024. So on systems with extremely high load, the observed
load average would actually fall back to 0 and shoot up again, creating
a kind of sawtooth graph.
Fix this by using 64-bit math internally, while still reporting the load
average to userspace as a 32-bit number.
Sponsored by: Axcient
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35134
Can MFC?
---
sys/kern/kern_synch.c | 9 +++++----
sys/kern/tty_info.c | 2 +-
sys/sys/param.h | 8 ++++----
3 files changed, 10 insertions(+), 9 deletions(-)
diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index e78878987b57..381d6315044c 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -87,7 +87,7 @@ struct loadavg averunnable =
* Constants for averages over 1, 5, and 15 minutes
* when sampling at 5 second intervals.
*/
-static fixpt_t cexp[3] = {
+static uint64_t cexp[3] = {
0.9200444146293232 * FSCALE, /* exp(-1/12) */
0.9834714538216174 * FSCALE, /* exp(-1/60) */
0.9944598480048967 * FSCALE, /* exp(-1/180) */
@@ -611,14 +611,15 @@ setrunnable(struct thread *td, int srqflags)
static void
loadav(void *arg)
{
- int i, nrun;
+ int i;
+ uint64_t nrun;
struct loadavg *avg;
- nrun = sched_load();
+ nrun = (uint64_t)sched_load();
avg = &averunnable;
for (i = 0; i < 3; i++)
- avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
+ avg->ldavg[i] = (cexp[i] * (uint64_t)avg->ldavg[i] +
nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
/*
diff --git a/sys/kern/tty_info.c b/sys/kern/tty_info.c
index 60675557e4ed..237aa47a18da 100644
--- a/sys/kern/tty_info.c
+++ b/sys/kern/tty_info.c
@@ -302,7 +302,7 @@ tty_info(struct tty *tp)
sbuf_set_drain(&sb, sbuf_tty_drain, tp);
/* Print load average. */
- load = (averunnable.ldavg[0] * 100 + FSCALE / 2) >> FSHIFT;
+ load = ((int64_t)averunnable.ldavg[0] * 100 + FSCALE / 2) >> FSHIFT;
sbuf_printf(&sb, "%sload: %d.%02d ", tp->t_column == 0 ? "" : "\n",
load / 100, load % 100);
diff --git a/sys/sys/param.h b/sys/sys/param.h
index 2d463b9ac7a2..b0b53f1a7776 100644
--- a/sys/sys/param.h
+++ b/sys/sys/param.h
@@ -361,12 +361,12 @@ __END_DECLS
* Scale factor for scaled integers used to count %cpu time and load avgs.
*
* The number of CPU `tick's that map to a unique `%age' can be expressed
- * by the formula (1 / (2 ^ (FSHIFT - 11))). The maximum load average that
- * can be calculated (assuming 32 bits) can be closely approximated using
- * the formula (2 ^ (2 * (16 - FSHIFT))) for (FSHIFT < 15).
+ * by the formula (1 / (2 ^ (FSHIFT - 11))). Since the intermediate
+ * calculation is done with 64-bit precision, the maximum load average that can
+ * be calculated is approximately 2^32 / FSCALE.
*
* For the scheduler to maintain a 1:1 mapping of CPU `tick' to `%age',
- * FSHIFT must be at least 11; this gives us a maximum load avg of ~1024.
+ * FSHIFT must be at least 11. This gives a maximum load avg of 2 million.
*/
#define FSHIFT 11 /* bits to right of fixed binary point
*/
#define FSCALE (1<<FSHIFT)
