From: Saravanan D <saravanand@fb.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: <x86@kernel.org>, <dave.hansen@linux.intel.com>,
<luto@kernel.org>, <peterz@infradead.org>, <corbet@lwn.net>,
<linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
<linux-doc@vger.kernel.org>, <linux-mm@kvack.org>,
<songliubraving@fb.com>
Subject: Re: [PATCH V5] x86/mm: Tracking linear mapping split events
Date: Thu, 28 Jan 2021 11:49:51 -0800 [thread overview]
Message-ID: <20210128194951.2sy3wt46ere4el3k@devvm1945.atn0.facebook.com> (raw)
In-Reply-To: <20210128150444.GY308988@casper.infradead.org>
Hi Mathew,
> Is this tracing of userspace programs causing splits, or is it kernel
> tracing? Also, we have lots of kinds of tracing these days; are you
> referring to kprobes? tracepoints? ftrace? Something else?
It has to be kernel tracing (kprobes, tracepoints) as we are dealing with
direct mapping splits.
Kernel's direct mapping
`` ffff888000000000 | -119.5 TB | ffffc87fffffffff | 64 TB | direct
mapping of all physical memory (page_offset_base)``
The kernel text range
``ffffffff80000000 | -2 GB | ffffffff9fffffff | 512 MB | kernel
text mapping, mapped to physical address 0``
Source : Documentation/x86/x86_64/mm.rst
Kernel code segment points to the same physical addresses already mapped
in the direct mapping range (0x20000000 = 512 MB)
When we enable kernel tracing, we would have to modify attributes/permissions
of the text segment pages that are direct mapped causing them to split.
When we track the direct_pages_count[] in arch/x86/mm/pat/set_memory.c
There are only splits from higher levels. They never coalesce back.
Splits when we turn on dynamic 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
....
Thanks,
Saravanan D
next prev parent reply other threads:[~2021-01-28 19:50 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <a936a943-9d8f-7e3c-af38-1c99ae176e1f@intel.com>
[not found] ` <20210128043547.1560435-1-saravanand@fb.com>
2021-01-28 4:51 ` [PATCH V4] x86/mm: Tracking linear mapping split events Matthew Wilcox
2021-01-28 10:49 ` [PATCH V5] " Saravanan D
2021-01-28 15:04 ` Matthew Wilcox
2021-01-28 19:49 ` Saravanan D [this message]
2021-01-28 16:33 ` Zi Yan
2021-01-28 16:41 ` Dave Hansen
2021-01-28 16:56 ` Zi Yan
2021-01-28 16:59 ` Song Liu
2021-01-28 19:17 ` Dave Hansen
2021-01-28 21:20 ` Saravanan D
2021-01-28 23:34 ` [PATCH V6] " Saravanan D
2021-01-28 23:41 ` Tejun Heo
2021-01-29 19:27 ` Johannes Weiner
2021-02-08 23:17 ` Saravanan D
2021-02-08 23:30 ` Dave Hansen
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