linux-kernel - Re: [PATCH 1/2 v5] SELinux: Reduce overhead of mls_level

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Date:	Mon, 08 Jul 2013 10:09:33 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Waiman Long <waiman.long@...com>
CC:	James Morris <james.l.morris@...cle.com>,
	Eric Paris <eparis@...isplace.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH 1/2 v5] SELinux: Reduce overhead of mls_level_isvalid()
 function call

On 07/05/2013 01:10 PM, Waiman Long wrote:
> On 06/11/2013 07:49 AM, Stephen Smalley wrote:
>> On 06/10/2013 01:55 PM, Waiman Long wrote:
>>> v4->v5:
>>>    - Fix scripts/checkpatch.pl warning.
>>>
>>> v3->v4:
>>>    - Merge the 2 separate while loops in ebitmap_contains() into
>>>      a single one.
>>>
>>> v2->v3:
>>>    - Remove unused local variables i, node from mls_level_isvalid().
>>>
>>> v1->v2:
>>>   - Move the new ebitmap comparison logic from mls_level_isvalid()
>>>     into the ebitmap_contains() helper function.
>>>   - Rerun perf and performance tests on the latest v3.10-rc4 kernel.
>>>
>>> While running the high_systime workload of the AIM7 benchmark on
>>> a 2-socket 12-core Westmere x86-64 machine running 3.10-rc4 kernel
>>> (with HT on), it was found that a pretty sizable amount of time was
>>> spent in the SELinux code. Below was the perf trace of the "perf
>>> record -a -s" of a test run at 1500 users:
>>>
>>>    5.04%            ls  [kernel.kallsyms]     [k] ebitmap_get_bit
>>>    1.96%            ls  [kernel.kallsyms]     [k] mls_level_isvalid
>>>    1.95%            ls  [kernel.kallsyms]     [k] find_next_bit
>>>
>>> The ebitmap_get_bit() was the hottest function in the perf-report
>>> output.  Both the ebitmap_get_bit() and find_next_bit() functions
>>> were, in fact, called by mls_level_isvalid(). As a result, the
>>> mls_level_isvalid() call consumed 8.95% of the total CPU time of
>>> all the 24 virtual CPUs which is quite a lot. The majority of the
>>> mls_level_isvalid() function invocations come from the socket creation
>>> system call.
>>>
>>> Looking at the mls_level_isvalid() function, it is checking to see
>>> if all the bits set in one of the ebitmap structure are also set in
>>> another one as well as the highest set bit is no bigger than the one
>>> specified by the given policydb data structure. It is doing it in
>>> a bit-by-bit manner. So if the ebitmap structure has many bits set,
>>> the iteration loop will be done many times.
>>>
>>> The current code can be rewritten to use a similar algorithm as the
>>> ebitmap_contains() function with an additional check for the
>>> highest set bit. The ebitmap_contains() function was extended to
>>> cover an optional additional check for the highest set bit, and the
>>> mls_level_isvalid() function was modified to call ebitmap_contains().
>>>
>>> With that change, the perf trace showed that the used CPU time drop
>>> down to just 0.08% (ebitmap_contains + mls_level_isvalid) of the
>>> total which is about 100X less than before.
>>>
>>>    0.07%            ls  [kernel.kallsyms]     [k] ebitmap_contains
>>>    0.05%            ls  [kernel.kallsyms]     [k] ebitmap_get_bit
>>>    0.01%            ls  [kernel.kallsyms]     [k] mls_level_isvalid
>>>    0.01%            ls  [kernel.kallsyms]     [k] find_next_bit
>>>
>>> The remaining ebitmap_get_bit() and find_next_bit() functions calls
>>> are made by other kernel routines as the new mls_level_isvalid()
>>> function will not call them anymore.
>>>
>>> This patch also improves the high_systime AIM7 benchmark result,
>>> though the improvement is not as impressive as is suggested by the
>>> reduction in CPU time spent in the ebitmap functions. The table below
>>> shows the performance change on the 2-socket x86-64 system (with HT
>>> on) mentioned above.
>>>
>>> +--------------+---------------+----------------+-----------------+
>>> |   Workload   | mean % change | mean % change  | mean % change   |
>>> |              | 10-100 users  | 200-1000 users | 1100-2000 users |
>>> +--------------+---------------+----------------+-----------------+
>>> | high_systime |     +0.1%     |     +0.9%      |     +2.6%       |
>>> +--------------+---------------+----------------+-----------------+
>>>
>>> Signed-off-by: Waiman Long <Waiman.Long@...com>
>>
>> Acked-by:  Stephen Smalley <sds@...ho.nsa.gov>
>>
>
> Thank for the Ack. Will that patch go into v3.11?

I hope so, but that's up to Eric Paris, who maintains the kernel tree 
for SELinux changes these days.


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