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Fri, 29 Jan 2021 11:27:15 -0800 (PST) Date: Fri, 29 Jan 2021 14:27:13 -0500 From: Johannes Weiner To: Saravanan D Cc: x86@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, willy@infradead.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, linux-mm@kvack.org, songliubraving@fb.com, tj@kernel.org Subject: Re: [PATCH V6] x86/mm: Tracking linear mapping split events Message-ID: References: <20210128212048.oopcyfdf4j2lc663@devvm1945.atn0.facebook.com> <20210128233430.1460964-1-saravanand@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210128233430.1460964-1-saravanand@fb.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Acked-by: Johannes Weiner