linux-kernel - Re: [PATCH 2/3] sched, timer: Use atomics for thread_group

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Date:	Wed, 15 Apr 2015 09:33:41 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jason Low <jason.low2@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	hideaki.kimura@...com, Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability


* Jason Low <jason.low2@...com> wrote:

> While running a database workload, we found a scalability issue with itimers.
> 
> Much of the problem was caused by the thread_group_cputimer spinlock.

So I'm fine with the basic principle, but in the hope that maybe 
posix-cpu-timers will grow similar optimizations in the future, it 
would help to have the new data type factored out better, not 
open-coded:

>  struct thread_group_cputimer {
> -	struct task_cputime cputime;
> +	atomic64_t utime;
> +	atomic64_t stime;
> +	atomic64_t sum_exec_runtime;
>  	int running;
> -	raw_spinlock_t lock;
>  };

So after your changes we still have a separate:

struct task_cputime {
        cputime_t utime;
        cputime_t stime;
        unsigned long long sum_exec_runtime;
};

Which then weirdly overlaps with a different structure on a different 
abstraction level:

 struct thread_group_cputimer {
	atomic64_t utime;
	atomic64_t stime;
	atomic64_t sum_exec_runtime;
 	int running;
 };

So I think it would be more obvious what's going on if we introduced 
an atomic task_cputime structure:

 struct task_cputime_atomic {
	atomic64_t utime;
	atomic64_t stime;
	atomic64_t sum_exec_runtime;
 };

and put that into 'struct thread_group_cputimer':

 struct thread_group_cputimer {
	struct task_cputime_atomic cputime_atomic;
  	int running;
 };

Maybe even factor out the main update and reading methods into 
expressively named helper inlines?

Thanks,

	Ingo
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