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Description of problem:

The `ss` utility is reading /proc/slabinfo every time it is executed
rather than when it actually needs the information.

For the customer system, which is running a low latency application
on the stock rhel6 kernel, this causes delay up around 100ms while
irq processing is blocked while gathering the information on slab
usage.

As it appears the the information gathered from /proc/slabinfo
is only used when running ss with the summary option (-s), based
on a suggestion from the customer i've done a patch to move
gathering of slabinfo so that it is only done when the information
is required.

Version-Release number of selected component (if applicable):

iproute-2.6.32-31.el6.x86_64

How reproducible:

100%

Steps to Reproduce:
1. Run ss with no options under strace
2. strace data shows /proc/slabinfo is read
3.

Actual results:

/proc/slabinfo is read every time

Expected results:

/proc/slabinfo is only read when the information is required

Additional info:

Created attachment 1103370 [details]
patch to prevent ss reading slabinfo unnecessarily

This upstream commit fixes the behaviour:

commit a221d621bb4af414b974ccc40bba26481337d7cf
Author: Bryton Lee <brytonlee01>
Date:   Thu Feb 12 14:16:04 2015 +0800

    prevent the read ahead of /proc/slabinfo in ss
    
    Signed-off-by: Bryton Lee <brytonlee01>

Fix verification is pretty simple using strace as suggested by Stuart:

Here's a test of the faulty version:

# strace -eopen ss 2>&1 | grep slabinfo
open("/proc/slabinfo", O_RDONLY)        = 3

With the patch from comment #3 applied, the above command does not produce any output.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2016-0895.html