linux-kernel - [RFC PATCH 0/2] ipc: do not hold ipc lock more than necessary

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Date:	Fri, 01 Mar 2013 16:16:32 -0800
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	linux-tip-commits@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH 0/2] ipc: do not hold ipc lock more than necessary

The following set of not-thoroughly-tested patches are based on the
discussion of holding the ipc lock unnecessarily, such as for permissions
and security checks:

https://lkml.org/lkml/2013/2/28/540

Patch 0/1: Introduces new functions, analogous to ipc_lock and ipc_lock_check
in the ipc utility code, allowing to obtain the ipc object without holding the lock.

Patch 0/2: Use the new functions and only acquire the ipc lock when needed.

With Rik's semop-multi.c microbenchmark we can see the following
results:

256 sems without patches:
+  59.40%            a.out  [kernel.kallsyms]  [k] _raw_spin_lock
+   6.14%            a.out  [kernel.kallsyms]  [k] sys_semtimedop
+   3.84%            a.out  [kernel.kallsyms]  [k] avc_has_perm_flags
+   3.64%            a.out  [kernel.kallsyms]  [k] __audit_syscall_exit
+   2.06%            a.out  [kernel.kallsyms]  [k] copy_user_enhanced_fast_string
+   1.86%            a.out  [kernel.kallsyms]  [k] ipc_lock
+   1.75%            a.out  [kernel.kallsyms]  [k] __audit_syscall_entry
+   1.69%            a.out  [kernel.kallsyms]  [k] ipc_has_perm.isra.21
+   1.47%            a.out  [kernel.kallsyms]  [k] do_smart_update
+   1.43%            a.out  [kernel.kallsyms]  [k] pid_vnr
+   1.39%            a.out  [kernel.kallsyms]  [k] try_atomic_semop.isra.5

total operations: 151452270, ops/sec 5048409

256 sems with patches:
+  17.47%            a.out  [kernel.kallsyms]  [k] _raw_spin_lock
+  11.08%            a.out  [kernel.kallsyms]  [k] sys_semtimedop
+   8.81%            a.out  [kernel.kallsyms]  [k] avc_has_perm_flags
+   7.96%            a.out  [kernel.kallsyms]  [k] ipc_has_perm.isra.21
+   6.50%            a.out  [kernel.kallsyms]  [k] __audit_syscall_exit
+   4.67%            a.out  [kernel.kallsyms]  [k] ipc_obtain_object_check
+   4.19%            a.out  [kernel.kallsyms]  [k] ipcperms
+   3.75%            a.out  [kernel.kallsyms]  [k] copy_user_enhanced_fast_string
+   3.38%            a.out  [kernel.kallsyms]  [k] system_call
+   3.05%            a.out  [kernel.kallsyms]  [k] try_atomic_semop.isra.5
+   2.70%            a.out  [kernel.kallsyms]  [k] do_smart_update
+   2.60%            a.out  [kernel.kallsyms]  [k] __audit_syscall_entry

total operations: 266502912, ops/sec 8883430

While the _raw_spin_lock time is drastically reduced, others do increase.
This results in an overall speedup of ~1.7x regarding ops/sec.


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