Re: [PATCH V5] x86/mm: Tracking linear mapping split events

Zi Yan
2021-01-28T16:35:22Z

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On 28 Jan 2021, at 5:49, Saravanan D wrote:

> To help with debugging the sluggishness caused by TLB miss/reload,
> we introduce monotonic lifetime hugepage split event counts since
> system state: SYSTEM_RUNNING to be displayed as part of
> /proc/vmstat in x86 servers
>
> The lifetime split event information will be displayed at the bottom of
> /proc/vmstat
> ....
> swap_ra 0
> swap_ra_hit 0
> direct_map_level2_splits 94
> direct_map_level3_splits 4
> nr_unstable 0
> ....
>
> One of the many lasting (as we don't coalesce back) sources for huge page
> splits is tracing as the granular page attribute/permission changes would
> force the kernel to split code segments mapped to huge pages to smaller
> ones thereby increasing the probability of TLB miss/reload even after
> tracing has been stopped.

It is interesting to see this statement saying splitting kernel direct mappings
causes performance loss, when Zhengjun (cc’d) from Intel recently posted
a kernel direct mapping performance report[1] saying 1GB mappings are good
but not much better than 2MB and 4KB mappings.

I would love to hear the stories from both sides. Or maybe I misunderstand
anything.

[1]https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com/
>
> Documentation regarding linear mapping split events added to admin-guide
> as requested in V3 of the patch.
>
> Signed-off-by: Saravanan D <saravanand@fb.com>
> ---
> .../admin-guide/mm/direct_mapping_splits.rst | 59 +++++++++++++++++++
> Documentation/admin-guide/mm/index.rst | 1 +
> arch/x86/mm/pat/set_memory.c | 8 +++
> include/linux/vm_event_item.h | 4 ++
> mm/vmstat.c | 4 ++
> 5 files changed, 76 insertions(+)
> create mode 100644 Documentation/admin-guide/mm/direct_mapping_splits.rst
>
> diff --git a/Documentation/admin-guide/mm/direct_mapping_splits.rst b/Documentation/admin-guide/mm/direct_mapping_splits.rst
> new file mode 100644
> index 000000000000..298751391deb
> --- /dev/null
> +++ b/Documentation/admin-guide/mm/direct_mapping_splits.rst
> @@ -0,0 +1,59 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=====================
> +Direct Mapping Splits
> +=====================
> +
> +Kernel maps all of physical memory in linear/direct mapped pages with
> +translation of virtual kernel address to physical address is achieved
> +through a simple subtraction of offset. CPUs maintain a cache of these
> +translations on fast caches called TLBs. CPU architectures like x86 allow
> +direct mapping large portions of memory into hugepages (2M, 1G, etc) in
> +various page table levels.
> +
> +Maintaining huge direct mapped pages greatly reduces TLB miss pressure.
> +The splintering of huge direct pages into smaller ones does result in
> +a measurable performance hit caused by frequent TLB miss and reloads.
> +
> +One of the many lasting (as we don't coalesce back) sources for huge page
> +splits is tracing as the granular page attribute/permission changes would
> +force the kernel to split code segments mapped to hugepages to smaller
> +ones thus increasing the probability of TLB miss/reloads even after
> +tracing has been stopped.
> +
> +On x86 systems, we can track the splitting of huge direct mapped pages
> +through lifetime event counters in ``/proc/vmstat``
> +
> + direct_map_level2_splits xxx
> + direct_map_level3_splits yyy
> +
> +where:
> +
> +direct_map_level2_splits
> + are 2M/4M hugepage split events
> +direct_map_level3_splits
> + are 1G hugepage split events
> +
> +The distribution of direct mapped system memory in various page sizes
> +post splits can be viewed through ``/proc/meminfo`` whose output
> +will include the following lines depending upon supporting CPU
> +architecture
> +
> + DirectMap4k: xxxxx kB
> + DirectMap2M: yyyyy kB
> + DirectMap1G: zzzzz kB
> +
> +where:
> +
> +DirectMap4k
> + is the total amount of direct mapped memory (in kB)
> + accessed through 4k pages
> +DirectMap2M
> + is the total amount of direct mapped memory (in kB)
> + accessed through 2M pages
> +DirectMap1G
> + is the total amount of direct mapped memory (in kB)
> + accessed through 1G pages
> +
> +
> +-- Saravanan D, Jan 27, 2021
> diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
> index 4b14d8b50e9e..9439780f3f07 100644
> --- a/Documentation/admin-guide/mm/index.rst
> +++ b/Documentation/admin-guide/mm/index.rst
> @@ -38,3 +38,4 @@ the Linux memory management.
> soft-dirty
> transhuge
> userfaultfd
> + direct_mapping_splits
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 16f878c26667..a7b3c5f1d316 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -16,6 +16,8 @@
> #include <linux/pci.h>
> #include <linux/vmalloc.h>
> #include <linux/libnvdimm.h>
> +#include <linux/vmstat.h>
> +#include <linux/kernel.h>
>
> #include <asm/e820/api.h>
> #include <asm/processor.h>
> @@ -91,6 +93,12 @@ static void split_page_count(int level)
> return;
>
> direct_pages_count[level]--;
> + if (system_state == SYSTEM_RUNNING) {
> + if (level == PG_LEVEL_2M)
> + count_vm_event(DIRECT_MAP_LEVEL2_SPLIT);
> + else if (level == PG_LEVEL_1G)
> + count_vm_event(DIRECT_MAP_LEVEL3_SPLIT);
> + }
> direct_pages_count[level - 1] += PTRS_PER_PTE;
> }
>
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 18e75974d4e3..7c06c2bdc33b 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -120,6 +120,10 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> #ifdef CONFIG_SWAP
> SWAP_RA,
> SWAP_RA_HIT,
> +#endif
> +#ifdef CONFIG_X86
> + DIRECT_MAP_LEVEL2_SPLIT,
> + DIRECT_MAP_LEVEL3_SPLIT,
> #endif
> NR_VM_EVENT_ITEMS
> };
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index f8942160fc95..a43ac4ac98a2 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1350,6 +1350,10 @@ const char * const vmstat_text[] = {
> "swap_ra",
> "swap_ra_hit",
> #endif
> +#ifdef CONFIG_X86
> + "direct_map_level2_splits",
> + "direct_map_level3_splits",
> +#endif
> #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
> };
> #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA || CONFIG_MEMCG */
> --
> 2.24.1

—
Best Regards,
Yan Zi

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