Re: ps performance sucks (was Re: dcache_rcu [performance results])

[William Lee Irwin III <wli@holomorphy.com>]

On Thu, Nov 07, 2002 at 11:06:13PM +1100, Rusty Russell wrote:
> Now, according to wli, there's a real problem with starvation by saturating
> the read side of the tasklist_lock so eg. the write_lock_irq(&tasklist_lock)
> in exit.c's release_task causes a CPU to spin for ages (forever?) with
> interrupts off.  This needs fixing, be it RCU or making that particular
> lock give way to writers, or some other effect.

One way to at least "postpone" having to do things like making a fair
tasklist_lock is to make readers well-behaved. /proc/ is the worst
remaining offender left with its quadratic (!) get_pid_list(). After
"kernel, you're being bad and spinning in near-infinite loops with the
tasklist_lock readlocked" is (completely?) solved, then we can wait for
boxen with higher cpu counts to catch fire anyway when the arrival rate
of readers * hold time of readers > 1, which will happen because arrival
rates are O(cpus), and cpus will grow without bound as machines advance.

I'm not sure RCU would help this any; I'd be very much afraid of the
writes being postponed indefinitely or just too long in the presence
of what's essentially perpetually in-progress read access. Does RCU
have a guarantee of forward progress for writers?

Thanks,
Bill