linux-kernel - Re: posix-cpu-timers revamp

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Message-Id: <1206995072.14649.41.camel@bobble.smo.corp.google.com>
Date:	Mon, 31 Mar 2008 13:24:32 -0700
From:	Frank Mayhar <fmayhar@...gle.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: posix-cpu-timers revamp

Roland, I'm very much having to read between the lines of what you've
written.  And, obviously, getting it wrong at least half the time. :-)

So you've cleared part of my understanding with your latest email.
Here's what I've gotten from it:

        struct task_cputime {
        	cputime_t utime;		/* User time. */
        	cputime_t stime;		/* System time. */
        	unsigned long long sched_runtime; /* Scheduler time. */
        };

This is for both SMP and UP, defined before signal_struct in sched.h
(since that structure refers to this one).  Following that:

        struct thread_group_cputime;

Which is a forward reference to the real definition later in the file.
The inline functions depend on signal_struct and task_struct, so they
have to come after:

        #ifdef SMP

        struct thread_group_cputime {
        	struct task_cputime *totals;
        };

        < ... inline functions ... >

        #else /* SMP */

        struct thread_group_cputime {
        	struct task_cputime totals;
        };

        < ... inline functions ... >

        #endif

The SMP version is percpu, the UP version is just a substructure.  In
signal_struct itself, delete utime & stime, add
        struct thread_group_cputime cputime;

The inline functions include the ones you defined for UP plus equivalent
ones for SMP.  The SMP inlines check the percpu pointer
(sig->cputime.totals) and don't update if it's NULL.  One small
correction to one of your inlines, in thread_group_cputime:
                *cputime = sig->cputime;
should be
                *cputime = sig->cputime.totals;

A representative inline for SMP is:

        static inline void account_group_system_time(struct task_struct *task,
        					      cputime_t cputime)
        {
        	struct task_cputime *times;

        	if (!sig->cputime.totals)
        		return;
        	times = per_cpu_ptr(sig->cputime.totals, get_cpu());
        	times->stime = cputime_add(times->stime, cputime);
        	put_cpu_no_resched();
        }

To deal with the need for bookkeeping with multiple threads in the SMP
case (where there isn't a per-cpu structure until it's needed), I'll
allocate the per-cpu structure in __exit_signal() where the relevant
fields are updated.  I'll also allocate it where I do now, in
do_setitimer(), when needed.  The allocation will be a "return 0" for UP
and a call to "thread_group_times_alloc_smp()" (which lives in sched.c)
for SMP.

I'll also optimize run_posix_cpu_timers() as you suggest, and eliminate
rlim_expires.

Expect a new patch fairly soon.
-- 
Frank Mayhar <fmayhar@...gle.com>
Google, Inc.

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