* A question regarding TG load tracking functions in fair.c
@ 2014-02-28 12:04 Matthew Leach
2014-02-28 12:29 ` Peter Zijlstra
0 siblings, 1 reply; 2+ messages in thread
From: Matthew Leach @ 2014-02-28 12:04 UTC (permalink / raw)
To: Paul Turner; +Cc: linux-kernel, Dietmar Eggemann, Peter Ziljstra
Hi Paul,
I have a question regarding two lines of code in the functions
__update_cfs_rq_tg_load_contrib and __update_tg_runnable_avg. AFAICS
these functions update a load-tracking signal for the local RQ and an
aggregated signal in the TG for all RQs that belong to this TG. The
update is surrounded by conditions:
if (abs(contrib) > cfs_rq->tg_runnable_contrib / 64) {
for __update_tg_runnable_avg and
if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
for __update_cfs_rq_tg_load_contrib. Is this an optimisation to prevent
the signals being updated for small variations? Also, could you please
shed some light on the specific numbers used?
Thanks,
--
Matt
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: A question regarding TG load tracking functions in fair.c
2014-02-28 12:04 A question regarding TG load tracking functions in fair.c Matthew Leach
@ 2014-02-28 12:29 ` Peter Zijlstra
0 siblings, 0 replies; 2+ messages in thread
From: Peter Zijlstra @ 2014-02-28 12:29 UTC (permalink / raw)
To: Matthew Leach; +Cc: Paul Turner, linux-kernel, Dietmar Eggemann
On Fri, Feb 28, 2014 at 12:04:19PM +0000, Matthew Leach wrote:
> Hi Paul,
>
> I have a question regarding two lines of code in the functions
> __update_cfs_rq_tg_load_contrib and __update_tg_runnable_avg. AFAICS
> these functions update a load-tracking signal for the local RQ and an
> aggregated signal in the TG for all RQs that belong to this TG. The
> update is surrounded by conditions:
>
> if (abs(contrib) > cfs_rq->tg_runnable_contrib / 64) {
>
> for __update_tg_runnable_avg and
>
> if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
>
> for __update_cfs_rq_tg_load_contrib. Is this an optimisation to prevent
> the signals being updated for small variations? Also, could you please
> shed some light on the specific numbers used?
Yes, its an optimization; updating global state is expensive. I'll let
Paul explain the specific numbers.
^ permalink raw reply [flat|nested] 2+ messages in thread
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