- Fixes rounding error for initial value with CONFIG_HZ_300
- Fixes read from the file after reset to default (by writing val <= 0)
Cyril Hrubis (2):
sched/rt: Fix sysctl_sched_rr_timeslice intial value
sched/rt: sysctl_sched_rr_timeslice show default timeslice after reset
kernel/sched/rt.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
--
2.41.0
The sched_rr_timeslice can be reset to default by writing value that is
<= 0. However after reading from this file we always got the last value
written, which is not useful at all.
$ echo -1 > /proc/sys/kernel/sched_rr_timeslice_ms
$ cat /proc/sys/kernel/sched_rr_timeslice_ms
-1
Fix this by setting the variable that holds the sysctl file value to the
jiffies_to_msecs(RR_TIMESLICE) in case that <= 0 value was written.
Signed-off-by: Cyril Hrubis <[email protected]>
CC: Jiri Bohac <[email protected]>
---
kernel/sched/rt.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 185d3d749f6b..0597ba0f85ff 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -3062,6 +3062,9 @@ static int sched_rr_handler(struct ctl_table *table, int write, void *buffer,
sched_rr_timeslice =
sysctl_sched_rr_timeslice <= 0 ? RR_TIMESLICE :
msecs_to_jiffies(sysctl_sched_rr_timeslice);
+
+ if (sysctl_sched_rr_timeslice <= 0)
+ sysctl_sched_rr_timeslice = jiffies_to_msecs(RR_TIMESLICE);
}
mutex_unlock(&mutex);
--
2.41.0
Thre is 10% rounding error in the intial value of the
sysctl_sched_rr_timeslice with CONFIG_HZ_300=y.
This was found with LTP test sched_rr_get_interval01:
sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
What this test does is to compare the return value from the
sched_rr_get_interval() and the sched_rr_timeslice_ms sysctl file and
fails if they do not match.
The prolem it found is the intial sysctl file value which was computed as:
static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
which works fine as long as MSEC_PER_SEC is multiple of HZ, however it
introduces 10% rounding error for CONFIG_HZ_300:
(MSEC_PER_SEC / HZ) * (100 * HZ / 1000)
(1000 / 300) * (100 * 300 / 1000)
3 * 30 = 90
This can be easily fixed by reversing the order of the multiplication
and division. After this fix we get:
(MSEC_PER_SEC * (100 * HZ / 1000)) / HZ
(1000 * (100 * 300 / 1000)) / 300
(1000 * 30) / 300 = 100
Signed-off-by: Cyril Hrubis <[email protected]>
CC: Jiri Bohac <[email protected]>
---
kernel/sched/rt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 00e0e5074115..185d3d749f6b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -25,7 +25,7 @@ unsigned int sysctl_sched_rt_period = 1000000;
int sysctl_sched_rt_runtime = 950000;
#ifdef CONFIG_SYSCTL
-static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
+static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC * RR_TIMESLICE) / HZ;
static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
size_t *lenp, loff_t *ppos);
static int sched_rr_handler(struct ctl_table *table, int write, void *buffer,
--
2.41.0
Hi,
[ Cc Shile Zhang ]
> Thre is 10% rounding error in the intial value of the
> sysctl_sched_rr_timeslice with CONFIG_HZ_300=y.
> This was found with LTP test sched_rr_get_interval01:
> sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
> sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
> sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
> sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
> sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
> sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
> What this test does is to compare the return value from the
> sched_rr_get_interval() and the sched_rr_timeslice_ms sysctl file and
> fails if they do not match.
> The prolem it found is the intial sysctl file value which was computed as:
> static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
> which works fine as long as MSEC_PER_SEC is multiple of HZ, however it
> introduces 10% rounding error for CONFIG_HZ_300:
> (MSEC_PER_SEC / HZ) * (100 * HZ / 1000)
> (1000 / 300) * (100 * 300 / 1000)
> 3 * 30 = 90
> This can be easily fixed by reversing the order of the multiplication
> and division. After this fix we get:
> (MSEC_PER_SEC * (100 * HZ / 1000)) / HZ
> (1000 * (100 * 300 / 1000)) / 300
> (1000 * 30) / 300 = 100
> Signed-off-by: Cyril Hrubis <[email protected]>
> CC: Jiri Bohac <[email protected]>
> ---
> kernel/sched/rt.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 00e0e5074115..185d3d749f6b 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -25,7 +25,7 @@ unsigned int sysctl_sched_rt_period = 1000000;
> int sysctl_sched_rt_runtime = 950000;
> #ifdef CONFIG_SYSCTL
> -static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
> +static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC * RR_TIMESLICE) / HZ;
It looks like very old bug, from v4.11-rc1. I guess this should go to all stable
and LTS kernels.
Fixes: 975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds")
Reviewed-by: Petr Vorel <[email protected]>
Kind regards,
Petr
> static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
> size_t *lenp, loff_t *ppos);
> static int sched_rr_handler(struct ctl_table *table, int write, void *buffer,
On Wed, Jul 19, 2023 at 12:37:42PM +0200, Cyril Hrubis wrote:
> Thre is 10% rounding error in the intial value of the
> sysctl_sched_rr_timeslice with CONFIG_HZ_300=y.
>
> This was found with LTP test sched_rr_get_interval01:
>
> sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
> sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
> sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
> sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
> sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
> sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
>
> What this test does is to compare the return value from the
> sched_rr_get_interval() and the sched_rr_timeslice_ms sysctl file and
> fails if they do not match.
>
> The prolem it found is the intial sysctl file value which was computed as:
>
> static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
>
> which works fine as long as MSEC_PER_SEC is multiple of HZ, however it
> introduces 10% rounding error for CONFIG_HZ_300:
>
> (MSEC_PER_SEC / HZ) * (100 * HZ / 1000)
>
> (1000 / 300) * (100 * 300 / 1000)
>
> 3 * 30 = 90
>
> This can be easily fixed by reversing the order of the multiplication
> and division. After this fix we get:
>
> (MSEC_PER_SEC * (100 * HZ / 1000)) / HZ
>
> (1000 * (100 * 300 / 1000)) / 300
>
> (1000 * 30) / 300 = 100
>
> Signed-off-by: Cyril Hrubis <[email protected]>
> CC: Jiri Bohac <[email protected]>
Acked-by: Mel Gorman <[email protected]>
--
Mel Gorman
SUSE Labs
On Wed, Jul 19, 2023 at 12:37:43PM +0200, Cyril Hrubis wrote:
> The sched_rr_timeslice can be reset to default by writing value that is
> <= 0. However after reading from this file we always got the last value
> written, which is not useful at all.
>
> $ echo -1 > /proc/sys/kernel/sched_rr_timeslice_ms
> $ cat /proc/sys/kernel/sched_rr_timeslice_ms
> -1
>
> Fix this by setting the variable that holds the sysctl file value to the
> jiffies_to_msecs(RR_TIMESLICE) in case that <= 0 value was written.
>
> Signed-off-by: Cyril Hrubis <[email protected]>
> CC: Jiri Bohac <[email protected]>
Acked-by: Mel Gorman <[email protected]>
--
Mel Gorman
SUSE Labs
Tested-by: Petr Vorel <[email protected]>
Kind regards,
Petr
Reviewed-by: Petr Vorel <[email protected]>
Tested-by: Petr Vorel <[email protected]>
Kind regards,
Petr