Re: [PATCH V6] x86/mm: Tracking linear mapping split events

[Johannes Weiner]

On Thu, Jan 28, 2021 at 03:34:30PM -0800, Saravanan D wrote:
> To help with debugging the sluggishness caused by TLB miss/reload,
> we introduce monotonic hugepage [direct mapped] 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 sources of direct hugepage splits is kernel
> tracing (kprobes, tracepoints).
> 
> Note that the kernel's code segment [512 MB] points to the same
> physical addresses that have been already mapped in the kernel's
> direct mapping range.
> 
> Source : Documentation/x86/x86_64/mm.rst
> 
> When we enable kernel tracing, the kernel has to modify
> attributes/permissions
> of the text segment hugepages that are direct mapped causing them to
> split.
> 
> Kernel's direct mapped hugepages do not coalesce back after split and
> remain in place for the remainder of the lifetime.
> 
> An instance of direct page splits when we turn on
> dynamic kernel tracing
> ....
> cat /proc/vmstat | grep -i direct_map_level
> direct_map_level2_splits 784
> direct_map_level3_splits 12
> bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @ [pid, comm] =
> count(); }'
> cat /proc/vmstat | grep -i
> direct_map_level
> direct_map_level2_splits 789
> direct_map_level3_splits 12
> ....
> 
> Signed-off-by: Saravanan D <saravanand@xxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>