Messages in this thread | | | From | Valentin Schneider <> | Subject | [RFC PATCH 0/7] sched: Streamline select_task_rq() & select_task_rq_fair() | Date | Wed, 11 Dec 2019 16:43:54 +0000 |
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I've been staring at select_task_rq_fair() for some time now, and have come to "hate" the usage of the sd_flag parameter. It is used as both an indicator of the wakeup type (ttwu/fork/exec) and as a domain walk search target. CFS is the only class doing this, the other classes just need the wakeup type but get passed a domain flag instead.
This series gets rid of select_task_rq()'s sd_flag parameter and also tries to optimize select_task_rq_fair().
There's a few ugly things in there that could be improved with some extra lawnmowing, hence me sending this as an RFC.
This is based on v5.5-rc1.
Patches =======
Patches 1-2 involve getting rid of the sd_flag parameter for select_task_rq().
Patches 3-4 are extra cleanups in the select_task_rq_fair() region.
Patches 5-7 split the want_affine vs !want_affine paths of select_task_rq_fair(), which unearths a domain walk that can be cached. Patches 6 & 7 should be considered as a single patch, they are only split to hopefully ease reviewing.
Discussion points =================
The use of SD_LOAD_BALANCE forced me to do a few ugly things.
Patch 5 is only required because the domain walk in select_task_rq_fair() is ceiled by the presence of SD_LOAD_BALANCE. Thing is (I also ramble about this in the changelog of patch 7) AFAIK this flag is set unconditionally in sd_init() and the only way to clear it is via the sched_domain sysctl, which is a SCHED_DEBUG interface. I haven't found anything with cpusets that would clear it; AFAICT cpusets can end up attaching the NULL domain to some CPUs (with sched_load_balance=0) but that's as far as this goes:
$ git grep SD_LOAD_BALANCE include/linux/sched/topology.h:#define SD_LOAD_BALANCE 0x0001 /* Do load balancing on this domain. */ kernel/sched/fair.c: if (!(tmp->flags & SD_LOAD_BALANCE)) kernel/sched/fair.c: if ((sd->flags & SD_LOAD_BALANCE) && kernel/sched/fair.c: if (!(sd->flags & SD_LOAD_BALANCE)) kernel/sched/fair.c: if (!(sd->flags & SD_LOAD_BALANCE)) kernel/sched/topology.c: if (!(sd->flags & SD_LOAD_BALANCE)) { kernel/sched/topology.c: printk(KERN_ERR "ERROR: !SD_LOAD_BALANCE domain has parent"); kernel/sched/topology.c: if (sd->flags & (SD_LOAD_BALANCE | kernel/sched/topology.c: pflags &= ~(SD_LOAD_BALANCE | kernel/sched/topology.c: .flags = 1*SD_LOAD_BALANCE
On top of this, the sysctl interface can break the SD pointers cached in update_top_cache_domain() (again, see patch 7).
In short, we should: - Fix the stale sched_domain cached pointer issue by either making the sysctl sd->flags entry read-only, or have it cause a cached pointer update
- Remove SD_LOAD_BALANCE (or at the very least its usage sites). IMO, not doing load balance translates to not having a sched_domain.
I'll be happy to do either of these, but I wanted to gather opinions before that.
Testing =======
Performance has been rather annoying to measure. I used to have some improvements on Juno (between 1% to 2%) and just noise on Xeon with my previous base - some tip/sched/core around 5.4-rc7. This has somehow changed with v5.5-rc1. The baseline also got about 2% worse (on Juno), I'll go and try to bisect this but I've been sitting on this for a while so I wanted to toss it out.
Juno r0 -------
SD levels are {MC, DIE}. 500 iterations of $(hackbench):
| | -PATCH | +PATCH | DELTA (%) | |-------+----------+----------+-----------| | count | 500 | 500 | | | mean | 0.630625 | 0.628632 | -0.316 | | std | 0.022276 | 0.024253 | +8.875 | | min | 0.584000 | 0.580000 | -0.685 | | 50% | 0.629000 | 0.625000 | -0.636 | | 95% | 0.666000 | 0.674000 | +1.201 | | 99% | 0.703000 | 0.699000 | -0.569 | | max | 0.748000 | 0.722000 | -3.476 |
I also tried pinning that to a single type of CPUs to reduce the 'noise' of asymmetric scheduling, this is 500 iterations of $(taskset -c 0,3-5 hackbench) (IOW LITTLEs only):
| | -PATCH | +PATCH | DELTA (%) | |-------+----------+----------+-----------| | count | 500 | 500 | | | mean | 1.092194 | 1.084328 | -0.720 | | std | 0.046710 | 0.049637 | +6.266 | | min | 1.017000 | 1.004000 | -1.278 | | 50% | 1.079000 | 1.072000 | -0.649 | | 95% | 1.177100 | 1.173300 | -0.323 | | 99% | 1.273060 | 1.261130 | -0.937 | | max | 1.342000 | 1.305000 | -2.757 |
Dual-socket Xeon E5 -------------------
That is 10 cores w/ SMT2 per socket. SD levels are {SMT, MC, NUMA}. 500 iterations of $(hackbench -l 1000):
| | -PATCH | +PATCH | DELTA (%) | |-------+----------+----------+-----------| | count | 500 | 500 | | | mean | 0.949278 | 0.946502 | -0.292 | | std | 0.006402 | 0.006784 | +5.967 | | min | 0.924000 | 0.921000 | -0.325 | | 50% | 0.950000 | 0.947000 | -0.316 | | 95% | 0.959000 | 0.957000 | -0.209 | | 99% | 0.963000 | 0.960010 | -0.310 | | max | 0.966000 | 0.963000 | -0.311 |
Notes =====
One thing of note is that we are considering getting rid of SD_BALANCE_WAKE for asymmetric systems, which are the last remaining (upstream) users of that flag. This would lower the usefulness of this series; nevertheless I think the cleanup (or at least part of it) is worth it.
Valentin Schneider (7): sched: Add WF_TTWU, WF_EXEC wakeup flags sched: Kill select_task_rq()'s sd_flag parameter sched/fair: find_idlest_group(): Remove unused sd_flag parameter sched/fair: Dissociate wakeup decisions from SD flag value sched/topology: Make {lowest/highest}_flag_domain() work with > 1 flags sched/fair: Split select_task_rq_fair want_affine logic sched/topology: Define and use shortcut pointers for wakeup sd_flag scan
kernel/sched/core.c | 10 +++---- kernel/sched/deadline.c | 4 +-- kernel/sched/fair.c | 61 +++++++++++++++++++++++++--------------- kernel/sched/idle.c | 2 +- kernel/sched/rt.c | 4 +-- kernel/sched/sched.h | 36 ++++++++++++++++-------- kernel/sched/stop_task.c | 2 +- kernel/sched/topology.c | 20 ++++++++++--- 8 files changed, 91 insertions(+), 48 deletions(-)
-- 2.24.0
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