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Date: Wed, 8 May 2024 15:21:27 -0700 From: Ian Rogers <irogers@...gle.com> To: Ravi Bangoria <ravi.bangoria@....com> Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org, namhyung@...nel.org, swapnil.sapkal@....com, mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org, rostedt@...dmis.org, vincent.guittot@...aro.org, bristot@...hat.com, adrian.hunter@...el.com, james.clark@....com, kan.liang@...ux.intel.com, gautham.shenoy@....com, kprateek.nayak@....com, juri.lelli@...hat.com, yangjihong@...edance.com, linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org, santosh.shukla@....com, ananth.narayan@....com, sandipan.das@....com Subject: Re: [RFC 0/4] perf sched: Introduce schedstat tool On Tue, May 7, 2024 at 11:05 PM Ravi Bangoria <ravi.bangoria@....com> wrote: > > MOTIVATION > ---------- > > Existing `perf sched` is quite exhaustive and provides lot of insights > into scheduler behavior but it quickly becomes impractical to use for > long running or scheduler intensive workload. For ex, `perf sched record` > has ~7.77% overhead on hackbench (with 25 groups each running 700K loops > on a 2-socket 128 Cores 256 Threads 3rd Generation EPYC Server), and it > generates huge 56G perf.data for which perf takes ~137 mins to prepare > and write it to disk [1]. > > Unlike `perf sched record`, which hooks onto set of scheduler tracepoints > and generates samples on a tracepoint hit, `perf sched schedstat record` > takes snapshot of the /proc/schedstat file before and after the workload, > i.e. there is zero interference on workload run. Also, it takes very > minimal time to parse /proc/schedstat, convert it into perf samples and > save those samples into perf.data file. Result perf.data file is much > smaller. So, overall `perf sched schedstat record` is much more light- > weight compare to `perf sched record`. > > We, internally at AMD, have been using this (a variant of this, known as > "sched-scoreboard"[2]) and found it to be very useful to analyse impact > of any scheduler code changes[3][4]. > > Please note that, this is not a replacement of perf sched record/report. > The intended users of the new tool are scheduler developers, not regular > users. > > USAGE > ----- > > # perf sched schedstat record > # perf sched schedstat report > > Note: Although perf schedstat tool supports workload profiling syntax > (i.e. -- <workload> ), the recorded profile is still systemwide since > the /proc/schedstat is a systemwide file. > > HOW TO INTERPRET THE REPORT > --------------------------- > > The schedstat report starts with total time profiling was active in > terms of jiffies: > > ---------------------------------------------------------------------------------------------------- > Time elapsed (in jiffies) : 24537 > ---------------------------------------------------------------------------------------------------- > > Next is cpu scheduling statistics. These are simple diffs of > /proc/schedstat cpu lines along with description. The report also > prints % relative to base stat. > > In the example below, schedule() left the cpu0 idle 98.19% of the time. > 16.54% of total try_to_wake_up() was to wakeup local cpu. And, the total > waittime by tasks on cpu0 is 0.49% of the total runtime by tasks on the > same cpu. > > ---------------------------------------------------------------------------------------------------- > cpu: cpu0 > ---------------------------------------------------------------------------------------------------- > sched_yield() count : 0 > Legacy counter can be ignored : 0 > schedule() called : 17138 > schedule() left the processor idle : 16827 ( 98.19% ) > try_to_wake_up() was called : 508 > try_to_wake_up() was called to wake up the local cpu : 84 ( 16.54% ) > total runtime by tasks on this processor (in jiffies) : 2408959243 > total waittime by tasks on this processor (in jiffies) : 11731825 ( 0.49% ) > total timeslices run on this cpu : 311 > ---------------------------------------------------------------------------------------------------- > > Next is load balancing statistics. For each of the sched domains > (eg: `SMT`, `MC`, `DIE`...), the scheduler computes statistics under > the following three categories: > > 1) Idle Load Balance: Load balancing performed on behalf of a long > idling cpu by some other cpu. > 2) Busy Load Balance: Load balancing performed when the cpu was busy. > 3) New Idle Balance : Load balancing performed when a cpu just became > idle. > > Under each of these three categories, schedstat report provides > different load balancing statistics. Along with direct stats, the > report also contains derived metrics prefixed with *. Example: > > ---------------------------------------------------------------------------------------------------- > CPU 0 DOMAIN 0 > ---------------------------------------------------------------------------------------------------- > ------------------------------------------<Category idle>------------------------------------------- > load_balance() count on cpu idle : 50 $ 490.74 $ > load_balance() found balanced on cpu idle : 42 $ 584.21 $ > load_balance() move task failed on cpu idle : 8 $ 3067.12 $ > imbalance sum on cpu idle : 8 > pull_task() count on cpu idle : 0 > pull_task() when target task was cache-hot on cpu idle : 0 > load_balance() failed to find busier queue on cpu idle : 0 $ 0.00 $ > load_balance() failed to find busier group on cpu idle : 42 $ 584.21 $ > *load_balance() success count on cpu idle : 0 > *avg task pulled per successful lb attempt (cpu idle) : 0.00 > ------------------------------------------<Category busy>------------------------------------------- > load_balance() count on cpu busy : 2 $ 12268.50 $ > load_balance() found balanced on cpu busy : 2 $ 12268.50 $ > load_balance() move task failed on cpu busy : 0 $ 0.00 $ > imbalance sum on cpu busy : 0 > pull_task() count on cpu busy : 0 > pull_task() when target task was cache-hot on cpu busy : 0 > load_balance() failed to find busier queue on cpu busy : 0 $ 0.00 $ > load_balance() failed to find busier group on cpu busy : 1 $ 24537.00 $ > *load_balance() success count on cpu busy : 0 > *avg task pulled per successful lb attempt (cpu busy) : 0.00 > -----------------------------------------<Category newidle>----------------------------------------- > load_balance() count on cpu newly idle : 427 $ 57.46 $ > load_balance() found balanced on cpu newly idle : 382 $ 64.23 $ > load_balance() move task failed on cpu newly idle : 45 $ 545.27 $ > imbalance sum on cpu newly idle : 48 > pull_task() count on cpu newly idle : 0 > pull_task() when target task was cache-hot on cpu newly idle : 0 > load_balance() failed to find busier queue on cpu newly idle : 0 $ 0.00 $ > load_balance() failed to find busier group on cpu newly idle : 382 $ 64.23 $ > *load_balance() success count on cpu newly idle : 0 > *avg task pulled per successful lb attempt (cpu newly idle) : 0.00 > ---------------------------------------------------------------------------------------------------- > > Consider following line: > > load_balance() found balanced on cpu newly idle : 382 $ 64.23 $ > > While profiling was active, the load-balancer found 382 times the load > needs to be balanced on a newly idle CPU 0. Following value encapsulated > inside $ is average jiffies between two events (24537 / 382 = 64.23). > > Next is active_load_balance() stats. alb did not trigger while I ran > a workload so it's all 0s. > > ----------------------------------<Category active_load_balance()>---------------------------------- > active_load_balance() count : 0 > active_load_balance() move task failed : 0 > active_load_balance() successfully moved a task : 0 > ---------------------------------------------------------------------------------------------------- > > Next are sched_balance_exec() and sched_balance_fork() stats. They are > not used but we kept it in RFC just for legacy purpose. Unless opposed, > we plan to remove them in next revision. > > Next is wakeup statistics. For every domain, the report also shows > task-wakeup statistics. Example: > > --------------------------------------------<Wakeup Info>------------------------------------------- > try_to_wake_up() awoke a task that last ran on a diff cpu : 12070 > try_to_wake_up() moved task because cache-cold on own cpu : 3324 > try_to_wake_up() started passive balancing : 0 > ---------------------------------------------------------------------------------------------------- > > Same set of stats are reported for each cpu and each domain level. > > > TODO: > - This RFC series supports v15 layout of /proc/schedstat but v16 layout > is already being pushed upstream. We are planning to include v16 as > well in the next revision. > - Currently schedstat tool provides statistics of only one run but we > are planning to add `perf sched schedstat diff` which can compare > the data of two different runs (possibly good and bad) and highlight > where scheduler decisions are impacting workload performance. > - perf sched schedstat records /proc/schedstat which is a cpu and domain > level scheduler statistic. We are planning to add taskstat tool which > reads task stats from procfs and generate scheduler statistic report > at task granularity. this will probably a standalone tool, something > like `perf sched taskstat record/report`. > - /proc/schedstat shows cpumask in domain line to indicate a group of > cpus that belong to the domain. Since we are not using domain<->cpumask > data anywhere, we are not capturing it as part of perf sample. But we > are planning to include it in the next revision. > - We have tested the patch only on AMD machines, not on other platforms. This is great! Is it possible to add some basic shell script testing: https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/shell?h=perf-tools-next for the sake of coverage, other platform testing, etc. ? Thanks, Ian > - Some of the perf related features can be added to the schedstat sub- > command as well: for ex, pipemode, -p <pid> option to profile running > process etc. We are not planning to add TUI support however. > - Code changes are for RFC and thus not optimal. Some of the places > where code changes are not required for RFC are marked as /* FIXME */. > - Documenting details about schedstat tool in perf-sched man page will > also be done in next revision. > - Checkpatch warnings are ignored for now. > > Patches are prepared on perf-tools-next/perf-tools-next (37862d6fdced). > Although all changes are in tools/perf/, kernel is important since it's > using v15 of /proc/schedstat. > > [1] https://youtu.be/lg-9aG2ajA0?t=283 > [2] https://github.com/AMDESE/sched-scoreboard > [3] https://lore.kernel.org/lkml/c50bdbfe-02ce-c1bc-c761-c95f8e216ca0@amdcom/ > [4] https://lore.kernel.org/lkml/3e32bec6-5e59-c66a-7676-7d15df2c961c@amdcom/ > > Ravi Bangoria (2): > perf sched: Make `struct perf_sched sched` a global variable > perf sched: Add template code for schedstat subcommand > > Swapnil Sapkal (2): > perf sched schedstat: Add record and rawdump support > perf sched schedstat: Add support for report subcommand > > tools/lib/perf/Documentation/libperf.txt | 2 + > tools/lib/perf/Makefile | 2 +- > tools/lib/perf/include/perf/event.h | 37 ++ > .../lib/perf/include/perf/schedstat-cpu-v15.h | 22 + > .../perf/include/perf/schedstat-domain-v15.h | 121 ++++ > tools/perf/builtin-inject.c | 2 + > tools/perf/builtin-sched.c | 558 ++++++++++++++++-- > tools/perf/util/event.c | 54 ++ > tools/perf/util/event.h | 2 + > tools/perf/util/session.c | 44 ++ > tools/perf/util/synthetic-events.c | 170 ++++++ > tools/perf/util/synthetic-events.h | 4 + > tools/perf/util/tool.h | 4 +- > 13 files changed, 965 insertions(+), 57 deletions(-) > create mode 100644 tools/lib/perf/include/perf/schedstat-cpu-v15.h > create mode 100644 tools/lib/perf/include/perf/schedstat-domain-v15.h > > -- > 2.44.0 >
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