Yafang Shao <laoar.shao@xxxxxxxxx> writes:
> On Wed, Jul 3, 2024 at 9:57 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Yafang Shao <laoar.shao@xxxxxxxxx> writes:
>>
>> > On Tue, Jul 2, 2024 at 5:10 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >>
>> >> Yafang Shao <laoar.shao@xxxxxxxxx> writes:
>> >>
>> >> > On Tue, Jul 2, 2024 at 10:51 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> >> >>
>> >> >> On Mon, 1 Jul 2024 22:20:46 +0800 Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>> >> >>
>> >> >> > Currently, we're encountering latency spikes in our container environment
>> >> >> > when a specific container with multiple Python-based tasks exits. These
>> >> >> > tasks may hold the zone->lock for an extended period, significantly
>> >> >> > impacting latency for other containers attempting to allocate memory.
>> >> >>
>> >> >> Is this locking issue well understood? Is anyone working on it? A
>> >> >> reasonably detailed description of the issue and a description of any
>> >> >> ongoing work would be helpful here.
>> >> >
>> >> > In our containerized environment, we have a specific type of container
>> >> > that runs 18 processes, each consuming approximately 6GB of RSS. These
>> >> > processes are organized as separate processes rather than threads due
>> >> > to the Python Global Interpreter Lock (GIL) being a bottleneck in a
>> >> > multi-threaded setup. Upon the exit of these containers, other
>> >> > containers hosted on the same machine experience significant latency
>> >> > spikes.
>> >> >
>> >> > Our investigation using perf tracing revealed that the root cause of
>> >> > these spikes is the simultaneous execution of exit_mmap() by each of
>> >> > the exiting processes. This concurrent access to the zone->lock
>> >> > results in contention, which becomes a hotspot and negatively impacts
>> >> > performance. The perf results clearly indicate this contention as a
>> >> > primary contributor to the observed latency issues.
>> >> >
>> >> > + 77.02% 0.00% uwsgi [kernel.kallsyms]
>> >> > [k] mmput ▒
>> >> > - 76.98% 0.01% uwsgi [kernel.kallsyms]
>> >> > [k] exit_mmap ▒
>> >> > - 76.97% exit_mmap
>> >> > ▒
>> >> > - 58.58% unmap_vmas
>> >> > ▒
>> >> > - 58.55% unmap_single_vma
>> >> > ▒
>> >> > - unmap_page_range
>> >> > ▒
>> >> > - 58.32% zap_pte_range
>> >> > ▒
>> >> > - 42.88% tlb_flush_mmu
>> >> > ▒
>> >> > - 42.76% free_pages_and_swap_cache
>> >> > ▒
>> >> > - 41.22% release_pages
>> >> > ▒
>> >> > - 33.29% free_unref_page_list
>> >> > ▒
>> >> > - 32.37% free_unref_page_commit
>> >> > ▒
>> >> > - 31.64% free_pcppages_bulk
>> >> > ▒
>> >> > + 28.65% _raw_spin_lock
>> >> > ▒
>> >> > 1.28% __list_del_entry_valid
>> >> > ▒
>> >> > + 3.25% folio_lruvec_lock_irqsave
>> >> > ▒
>> >> > + 0.75% __mem_cgroup_uncharge_list
>> >> > ▒
>> >> > 0.60% __mod_lruvec_state
>> >> > ▒
>> >> > 1.07% free_swap_cache
>> >> > ▒
>> >> > + 11.69% page_remove_rmap
>> >> > ▒
>> >> > 0.64% __mod_lruvec_page_state
>> >> > - 17.34% remove_vma
>> >> > ▒
>> >> > - 17.25% vm_area_free
>> >> > ▒
>> >> > - 17.23% kmem_cache_free
>> >> > ▒
>> >> > - 17.15% __slab_free
>> >> > ▒
>> >> > - 14.56% discard_slab
>> >> > ▒
>> >> > free_slab
>> >> > ▒
>> >> > __free_slab
>> >> > ▒
>> >> > __free_pages
>> >> > ▒
>> >> > - free_unref_page
>> >> > ▒
>> >> > - 13.50% free_unref_page_commit
>> >> > ▒
>> >> > - free_pcppages_bulk
>> >> > ▒
>> >> > + 13.44% _raw_spin_lock
>> >> >
>> >> > By enabling the mm_page_pcpu_drain() we can find the detailed stack:
>> >> >
>> >> > <...>-1540432 [224] d..3. 618048.023883: mm_page_pcpu_drain:
>> >> > page=0000000035a1b0b7 pfn=0x11c19c72 order=0 migratetyp
>> >> > e=1
>> >> > <...>-1540432 [224] d..3. 618048.023887: <stack trace>
>> >> > => free_pcppages_bulk
>> >> > => free_unref_page_commit
>> >> > => free_unref_page_list
>> >> > => release_pages
>> >> > => free_pages_and_swap_cache
>> >> > => tlb_flush_mmu
>> >> > => zap_pte_range
>> >> > => unmap_page_range
>> >> > => unmap_single_vma
>> >> > => unmap_vmas
>> >> > => exit_mmap
>> >> > => mmput
>> >> > => do_exit
>> >> > => do_group_exit
>> >> > => get_signal
>> >> > => arch_do_signal_or_restart
>> >> > => exit_to_user_mode_prepare
>> >> > => syscall_exit_to_user_mode
>> >> > => do_syscall_64
>> >> > => entry_SYSCALL_64_after_hwframe
>> >> >
>> >> > The servers experiencing these issues are equipped with impressive
>> >> > hardware specifications, including 256 CPUs and 1TB of memory, all
>> >> > within a single NUMA node. The zoneinfo is as follows,
>> >> >
>> >> > Node 0, zone Normal
>> >> > pages free 144465775
>> >> > boost 0
>> >> > min 1309270
>> >> > low 1636587
>> >> > high 1963904
>> >> > spanned 564133888
>> >> > present 296747008
>> >> > managed 291974346
>> >> > cma 0
>> >> > protection: (0, 0, 0, 0)
>> >> > ...
>> >> > ...
>> >> > pagesets
>> >> > cpu: 0
>> >> > count: 2217
>> >> > high: 6392
>> >> > batch: 63
>> >> > vm stats threshold: 125
>> >> > cpu: 1
>> >> > count: 4510
>> >> > high: 6392
>> >> > batch: 63
>> >> > vm stats threshold: 125
>> >> > cpu: 2
>> >> > count: 3059
>> >> > high: 6392
>> >> > batch: 63
>> >> >
>> >> > ...
>> >> >
>> >> > The high is around 100 times the batch size.
>> >> >
>> >> > We also traced the latency associated with the free_pcppages_bulk()
>> >> > function during the container exit process:
>> >> >
>> >> > 19:48:54
>> >> > nsecs : count distribution
>> >> > 0 -> 1 : 0 | |
>> >> > 2 -> 3 : 0 | |
>> >> > 4 -> 7 : 0 | |
>> >> > 8 -> 15 : 0 | |
>> >> > 16 -> 31 : 0 | |
>> >> > 32 -> 63 : 0 | |
>> >> > 64 -> 127 : 0 | |
>> >> > 128 -> 255 : 0 | |
>> >> > 256 -> 511 : 148 |***************** |
>> >> > 512 -> 1023 : 334 |****************************************|
>> >> > 1024 -> 2047 : 33 |*** |
>> >> > 2048 -> 4095 : 5 | |
>> >> > 4096 -> 8191 : 7 | |
>> >> > 8192 -> 16383 : 12 |* |
>> >> > 16384 -> 32767 : 30 |*** |
>> >> > 32768 -> 65535 : 21 |** |
>> >> > 65536 -> 131071 : 15 |* |
>> >> > 131072 -> 262143 : 27 |*** |
>> >> > 262144 -> 524287 : 84 |********** |
>> >> > 524288 -> 1048575 : 203 |************************ |
>> >> > 1048576 -> 2097151 : 284 |********************************** |
>> >> > 2097152 -> 4194303 : 327 |*************************************** |
>> >> > 4194304 -> 8388607 : 215 |************************* |
>> >> > 8388608 -> 16777215 : 116 |************* |
>> >> > 16777216 -> 33554431 : 47 |***** |
>> >> > 33554432 -> 67108863 : 8 | |
>> >> > 67108864 -> 134217727 : 3 | |
>> >> >
>> >> > avg = 3066311 nsecs, total: 5887317501 nsecs, count: 1920
>> >> >
>> >> > The latency can reach tens of milliseconds.
>> >> >
>> >> > By adjusting the vm.percpu_pagelist_high_fraction parameter to set the
>> >> > minimum pagelist high at 4 times the batch size, we were able to
>> >> > significantly reduce the latency associated with the
>> >> > free_pcppages_bulk() function during container exits.:
>> >> >
>> >> > nsecs : count distribution
>> >> > 0 -> 1 : 0 | |
>> >> > 2 -> 3 : 0 | |
>> >> > 4 -> 7 : 0 | |
>> >> > 8 -> 15 : 0 | |
>> >> > 16 -> 31 : 0 | |
>> >> > 32 -> 63 : 0 | |
>> >> > 64 -> 127 : 0 | |
>> >> > 128 -> 255 : 120 | |
>> >> > 256 -> 511 : 365 |* |
>> >> > 512 -> 1023 : 201 | |
>> >> > 1024 -> 2047 : 103 | |
>> >> > 2048 -> 4095 : 84 | |
>> >> > 4096 -> 8191 : 87 | |
>> >> > 8192 -> 16383 : 4777 |************** |
>> >> > 16384 -> 32767 : 10572 |******************************* |
>> >> > 32768 -> 65535 : 13544 |****************************************|
>> >> > 65536 -> 131071 : 12723 |************************************* |
>> >> > 131072 -> 262143 : 8604 |************************* |
>> >> > 262144 -> 524287 : 3659 |********** |
>> >> > 524288 -> 1048575 : 921 |** |
>> >> > 1048576 -> 2097151 : 122 | |
>> >> > 2097152 -> 4194303 : 5 | |
>> >> >
>> >> > avg = 103814 nsecs, total: 5805802787 nsecs, count: 55925
>> >> >
>> >> > After successfully tuning the vm.percpu_pagelist_high_fraction sysctl
>> >> > knob to set the minimum pagelist high at a level that effectively
>> >> > mitigated latency issues, we observed that other containers were no
>> >> > longer experiencing similar complaints. As a result, we decided to
>> >> > implement this tuning as a permanent workaround and have deployed it
>> >> > across all clusters of servers where these containers may be deployed.
>> >>
>> >> Thanks for your detailed data.
>> >>
>> >> IIUC, the latency of free_pcppages_bulk() during process exiting
>> >> shouldn't be a problem?
>> >
>> > Right. The problem arises when the process holds the lock for too
>> > long, causing other processes that are attempting to allocate memory
>> > to experience delays or wait times.
>> >
>> >> Because users care more about the total time of
>> >> process exiting, that is, throughput. And I suspect that the zone->lock
>> >> contention and page allocating/freeing throughput will be worse with
>> >> your configuration?
>> >
>> > While reducing throughput may not be a significant concern due to the
>> > minimal difference, the potential for latency spikes, a crucial metric
>> > for assessing system stability, is of greater concern to users. Higher
>> > latency can lead to request errors, impacting the user experience.
>> > Therefore, maintaining stability, even at the cost of slightly lower
>> > throughput, is preferable to experiencing higher throughput with
>> > unstable performance.
>> >
>> >>
>> >> But the latency of free_pcppages_bulk() and page allocation in other
>> >> processes is a problem. And your configuration can help it.
>> >>
>> >> Another choice is to change CONFIG_PCP_BATCH_SCALE_MAX. In that way,
>> >> you have a normal PCP size (high) but smaller PCP batch. I guess that
>> >> may help both latency and throughput in your system. Could you give it
>> >> a try?
>> >
>> > Currently, our kernel does not include the CONFIG_PCP_BATCH_SCALE_MAX
>> > configuration option. However, I've observed your recent improvements
>> > to the zone->lock mechanism, particularly commit 52166607ecc9 ("mm:
>> > restrict the pcp batch scale factor to avoid too long latency"), which
>> > has prompted me to experiment with manually setting the
>> > pcp->free_factor to zero. While this adjustment provided some
>> > improvement, the results were not as significant as I had hoped.
>> >
>> > BTW, perhaps we should consider the implementation of a sysctl knob as
>> > an alternative to CONFIG_PCP_BATCH_SCALE_MAX? This would allow users
>> > to more easily adjust it.
>>
>> If you cannot test upstream behavior, it's hard to make changes to
>> upstream. Could you find a way to do that?
>
> I'm afraid I can't run an upstream kernel in our production environment :(
> Lots of code changes have to be made.
Understand. Can you find a way to test upstream behavior, not upstream
kernel exactly? Or test the upstream kernel but in a similar but not
exactly production environment.
>> IIUC, PCP high will not influence allocate/free latency, PCP batch will.
>
> It seems incorrect.
> Looks at the code in free_unref_page_commit() :
>
> if (pcp->count >= high) {
> free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
> pcp, pindex);
> }
>
> And nr_pcp_free() :
> min_nr_free = batch;
> max_nr_free = high - batch;
>
> batch = clamp_t(int, pcp->free_count, min_nr_free, max_nr_free);
> return batch;
>
> The 'batch' is not a fixed value but changed dynamically, isn't it ?
Sorry, my words were confusing. For 'batch', I mean the value of the
"count" parameter of free_pcppages_bulk() actually. For example, if we
change CONFIG_PCP_BATCH_SCALE_MAX, we restrict that.
>> Your configuration will influence PCP batch via configuration PCP high.
>> So, it may be reasonable to find a way to adjust PCP batch directly.
>> But, we need practical requirements and test methods first.
>>
--
Best Regards,
Huang, Ying
