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Description of problem:

When matching on process owner, command name, and possibly in some other situations, pgrep and pkill enable too many / incorrect PROC_FILL* flags.  This can lead to poor performance and unnecessary blocking.

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

procps-3.2.8-25.el6.x86_64

How reproducible:

Always

Steps to Reproduce:

Test 1:
strace -e trace=file pgrep httpd

Test 2:
strace -e trace=file pgrep -u apache

Actual results:

In test 1, observe that /proc/<pid>/status and /proc/<pid>/cmdline are opened for processes.

In test 2, observe that /proc/<pid>/status is opened for all processes owned by apache

Expected results:

In the first case, there is no need to open /proc/<pid>/cmdline, and it may be faster to use /proc/<pid>/stat rather than status (I'm unsure on the second point).

In the second case, neither /proc/<pid>/stat nor /proc/<pid>/status is required.

Additional info:

I've tested changing the flag logic in pgrep.c:do_openproc, and the change seems trivial, but the exact patch depends on whether stat or status is really the preferred way to match process names.

Created attachment 912911 [details]
proposed patch

This is what I tested.

Created attachment 957225 [details]
amended patch

there was a typo in the original patch here (PROC_STAT should have been PROC_FILLSTAT).  Updated.

The proposed modification can affect the internal logic and cause regressions in corner case scenarios. The internal filling/matching logic will have to be redesigned completely as some of the fill flags lead to duplicating/overwriting of the fields and it's difficult to track that. We (reporter and I) decided to wait till the mess is resolved upstream in order to avoid breaking things.