Re: [PATCH 4/4] sched/fair: Add document for burstable CFS bandwidth control

[Randy Dunlap]

Hi--

Some comments below:

On 1/20/21 4:27 AM, Huaixin Chang wrote:
> Basic description of usage and effect for CFS Bandwidth Control Burst.
> 
> Signed-off-by: Huaixin Chang <changhuaixin@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Shanpei Chen <shanpeic@xxxxxxxxxxxxxxxxx>
> ---
>  Documentation/scheduler/sched-bwc.rst | 70 +++++++++++++++++++++++++++++++++--
>  1 file changed, 66 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
> index 9801d6b284b1..2214ecaad393 100644
> --- a/Documentation/scheduler/sched-bwc.rst
> +++ b/Documentation/scheduler/sched-bwc.rst
> @@ -21,18 +21,46 @@ cfs_quota units at each period boundary. As threads consume this bandwidth it
>  is transferred to cpu-local "silos" on a demand basis. The amount transferred
>  within each of these updates is tunable and described as the "slice".
>  
> +By default, CPU bandwidth consumption is strictly limited to quota within each
> +given period. For the sequence of CPU usage u_i served under CFS bandwidth
> +control, if for any j <= k N(j,k) is the number of periods from u_j to u_k:
> +
> +        u_j+...+u_k <= quota * N(j,k)
> +
> +For a bursty sequence among which interval u_j...u_k are at the peak, CPU
> +requests might have to wait for more periods to replenish enough quota.
> +Otherwise, larger quota is required.
> +
> +With "burst" buffer, CPU requests might be served as long as:
> +
> +        u_j+...+u_k <= B_j + quota * N(j,k)
> +
> +if for any j <= k N(j,k) is the number of periods from u_j to u_k and B_j is
> +the accumulated quota from previous periods in burst buffer serving u_j.
> +Burst buffer helps in that serving whole bursty CPU requests without throttling
> +them can be done with moderate quota setting and accumulated quota in burst
> +buffer, if:
> +
> +        u_0+...+u_n <= B_0 + quota * N(0,n)
> +
> +where B_0 is the initial state of burst buffer. The maximum accumulated quota in
> +the burst buffer is capped by burst. With proper burst setting, the available
> +bandwidth is still determined by quota and period on the long run.
> +
>  Management
>  ----------
> -Quota and period are managed within the cpu subsystem via cgroupfs.
> +Quota, period and burst are managed within the cpu subsystem via cgroupfs.
>  
> -cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
> +cpu.cfs_quota_us: run-time replenished within a period (in microseconds)
>  cpu.cfs_period_us: the length of a period (in microseconds)
> +cpu.cfs_burst_us: the maximum accumulated run-time (in microseconds)
>  cpu.stat: exports throttling statistics [explained further below]
>  
>  The default values are::
>  
>  	cpu.cfs_period_us=100ms
> -	cpu.cfs_quota=-1
> +	cpu.cfs_quota_us=-1
> +	cpu.cfs_burst_us=0
>  
>  A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
>  bandwidth restriction in place, such a group is described as an unconstrained
> @@ -48,6 +76,11 @@ more detail below.
>  Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
>  and return the group to an unconstrained state once more.
>  
> +A value of 0 for cpu.cfs_burst_us indicates that the group can not accumulate
> +any unused bandwidth. It makes the traditional bandwidth control behavior for
> +CFS unchanged. Writing any (valid) positive value(s) into cpu.cfs_burst_us
> +will enact the cap on unused bandwidth accumulation.
> +
>  Any updates to a group's bandwidth specification will result in it becoming
>  unthrottled if it is in a constrained state.
>  
> @@ -65,9 +98,21 @@ This is tunable via procfs::
>  Larger slice values will reduce transfer overheads, while smaller values allow
>  for more fine-grained consumption.
>  
> +There is also a global switch to turn off burst for all groups::
> +       /proc/sys/kernel/sched_cfs_bw_burst_enabled (default=1)
> +
> +By default it is enabled. Write 0 values means no accumulated CPU time can be

                             Writing a 0 value means

> +used for any group, even if cpu.cfs_burst_us is configured.
> +
> +Sometimes users might want a group to burst without accumulation. This is
> +tunable via::
> +       /proc/sys/kernel/sched_cfs_bw_burst_onset_percent (default=0)
> +
> +Up to 100% runtime of cpu.cfs_burst_us might be given on setting bandwidth.
> +
>  Statistics
>  ----------
> -A group's bandwidth statistics are exported via 3 fields in cpu.stat.
> +A group's bandwidth statistics are exported via 6 fields in cpu.stat.
>  
>  cpu.stat:
>  
> @@ -75,6 +120,11 @@ cpu.stat:
>  - nr_throttled: Number of times the group has been throttled/limited.
>  - throttled_time: The total time duration (in nanoseconds) for which entities
>    of the group have been throttled.
> +- current_bw: Current runtime in global pool.
> +- nr_burst: Number of periods burst occurs.
> +- burst_time: Cumulative wall-time that any cpus has used above quota in

                                               CPUs have used

> +  respective periods
> +
>  
>  This interface is read-only.
>  
> @@ -172,3 +222,15 @@ Examples
>  
>     By using a small period here we are ensuring a consistent latency
>     response at the expense of burst capacity.
> +
> +4. Limit a group to 20% of 1 CPU, and allow accumulate up to 60% of 1 CPU
> +   addtionally, in case accumulation has been done.

      additionally,

> +
> +   With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU.
> +   And 30ms burst will be equivalent to 60% of 1 CPU.
> +
> +	# echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
> +	# echo 50000 > cpu.cfs_period_us /* period = 50ms */
> +	# echo 30000 > cpu.cfs_burst_us /* burst = 30ms */
> +
> +   Larger buffer setting allows greater burst capacity.
> 


HTH.
-- 
~Randy
"He closes his eyes and drops the goggles.  You can't get hurt
by looking at a bitmap.  Or can you?"
(Neal Stephenson: Snow Crash)