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Date:   Fri, 23 Apr 2021 13:47:53 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     lkp <oliver.sang@intel.com>
Cc:     David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        lkp@intel.com, ying.huang@intel.com, feng.tang@intel.com,
        zhengjun.xing@intel.com
Subject: Re: [mm/writeback]  e5dbd33218:  will-it-scale.per_process_ops -3.8%
 regression
Message-ID: <20210423124753.GA235567@casper.infradead.org>
References: <20210423054601.GC13944@xsang-OptiPlex-9020>
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On Fri, Apr 23, 2021 at 01:46:01PM +0800, kernel test robot wrote:
> FYI, we noticed a -3.8% regression of will-it-scale.per_process_ops due to commit:
> commit: e5dbd33218bd8d87ab69f730ab90aed5fab7eb26 ("mm/writeback: Add wait_on_page_writeback_killable")

That commit just adds a function.  It doesn't add any callers.  It must
just be moving something around ...

> 39f985c8f667c80a e5dbd33218bd8d87ab69f730ab9 
> ---------------- --------------------------- 
>          %stddev     %change         %stddev
>              \          |                \  
>    9359770            -3.8%    9001769        will-it-scale.16.processes
>     584985            -3.8%     562610        will-it-scale.per_process_ops
>    9359770            -3.8%    9001769        will-it-scale.workload
>      15996            -1.2%      15811        proc-vmstat.nr_kernel_stack
>      23577 ? 10%     +18.5%      27937 ?  7%  softirqs.CPU48.SCHED
>       5183 ? 41%     +47.2%       7630 ?  7%  interrupts.CPU1.NMI:Non-maskable_interrupts
>       5183 ? 41%     +47.2%       7630 ?  7%  interrupts.CPU1.PMI:Performance_monitoring_interrupts
>      54.33 ? 12%     +18.4%      64.33 ?  7%  perf-sched.wait_and_delay.count.schedule_hrtimeout_range_clock.poll_schedule_timeout.constprop.0.do_sys_poll
>     153.34 ? 24%     -45.9%      83.00 ? 25%  perf-sched.wait_and_delay.max.ms.schedule_timeout.rcu_gp_kthread.kthread.ret_from_fork
>     153.33 ? 24%     -45.9%      82.99 ? 25%  perf-sched.wait_time.max.ms.schedule_timeout.rcu_gp_kthread.kthread.ret_from_fork
>  2.424e+10            -3.8%  2.332e+10        perf-stat.i.branch-instructions
>       0.47            +3.7%       0.48        perf-stat.i.cpi
>  2.529e+10            -4.0%  2.428e+10        perf-stat.i.dTLB-loads
>   1.15e+10            -3.8%  1.106e+10        perf-stat.i.dTLB-stores
>   54249733            -4.8%   51627939        perf-stat.i.iTLB-load-misses
>  1.004e+11            -3.8%  9.661e+10        perf-stat.i.instructions
>       2.15            -3.6%       2.07        perf-stat.i.ipc
>     693.66            -3.9%     666.70        perf-stat.i.metric.M/sec
>       0.46            +3.7%       0.48        perf-stat.overall.cpi
>       2.15            -3.6%       2.08        perf-stat.overall.ipc
>  2.416e+10            -3.8%  2.324e+10        perf-stat.ps.branch-instructions
>   2.52e+10            -4.0%  2.419e+10        perf-stat.ps.dTLB-loads
>  1.146e+10            -3.8%  1.102e+10        perf-stat.ps.dTLB-stores
>   54065825            -4.8%   51454019        perf-stat.ps.iTLB-load-misses
>  1.001e+11            -3.8%  9.628e+10        perf-stat.ps.instructions
>  3.025e+13            -3.9%  2.908e+13        perf-stat.total.instructions
>       0.89 ? 14%      -0.1        0.77 ? 11%  perf-profile.calltrace.cycles-pp.atime_needs_update.touch_atime.shmem_mmap.mmap_region.do_mmap
>       0.14 ? 13%      -0.1        0.04 ? 71%  perf-profile.children.cycles-pp.common_mmap
>       0.61 ? 12%      -0.1        0.52 ? 12%  perf-profile.children.cycles-pp.common_file_perm
>       0.21 ?  8%      -0.0        0.17 ? 11%  perf-profile.children.cycles-pp.vma_set_page_prot
>       0.12 ?  8%      -0.0        0.09 ? 12%  perf-profile.children.cycles-pp.blocking_notifier_call_chain
>       0.12 ? 14%      -0.0        0.09 ? 15%  perf-profile.children.cycles-pp.get_mmap_base
>       0.09 ?  8%      -0.0        0.07 ? 11%  perf-profile.children.cycles-pp.vm_pgprot_modify
>       0.13 ? 15%      +0.1        0.19 ?  8%  perf-profile.children.cycles-pp.cap_capable
>       0.03 ?102%      +0.1        0.12 ? 12%  perf-profile.children.cycles-pp.munmap@plt
>       0.14 ? 13%      +0.1        0.24 ?  6%  perf-profile.children.cycles-pp.testcase
>       0.33 ? 10%      -0.1        0.23 ? 10%  perf-profile.self.cycles-pp.cap_vm_enough_memory
>       0.13 ? 11%      -0.1        0.03 ?100%  perf-profile.self.cycles-pp.common_mmap
>       0.48 ? 12%      -0.1        0.41 ? 12%  perf-profile.self.cycles-pp.common_file_perm
>       0.49 ? 12%      -0.1        0.43 ? 13%  perf-profile.self.cycles-pp.vm_area_alloc
>       0.12 ?  8%      -0.0        0.09 ? 12%  perf-profile.self.cycles-pp.blocking_notifier_call_chain
>       0.12 ? 13%      -0.0        0.09 ? 14%  perf-profile.self.cycles-pp.get_mmap_base
>       0.11 ?  8%      +0.0        0.16 ? 10%  perf-profile.self.cycles-pp.__x64_sys_munmap
>       0.11 ? 14%      +0.1        0.18 ?  8%  perf-profile.self.cycles-pp.cap_capable
>       0.12 ? 11%      +0.1        0.20 ?  6%  perf-profile.self.cycles-pp.testcase
>       0.01 ?223%      +0.1        0.11 ? 13%  perf-profile.self.cycles-pp.munmap@plt

I'm struggling to see anything in that that says anything other than
"we did 3-4% less work".  Maybe someone else has something useful to
say about it?