[Devel] Re: [RFC] kernel/pid.c pid allocation wierdness

[Eric Dumazet]

On Friday 16 March 2007 11:57, Pavel Emelianov wrote:
> Oleg Nesterov wrote:
> > On 03/14, Eric W. Biederman wrote:
> >> Pavel Emelianov <xemul at sw.ru> writes:
> >>> Hi.
> >>>
> >>> I'm looking at how alloc_pid() works and can't understand
> >>> one (simple/stupid) thing.
> >>>
> >>> It first kmem_cache_alloc()-s a strct pid, then calls
> >>> alloc_pidmap() and at the end it taks a global pidmap_lock()
> >>> to add new pid to hash.
> >
> > We need some global lock. pidmap_lock is already here, and it is
> > only used to protect pidmap->page allocation. Iow, it is almost
> > unused. So it was very natural to re-use it while implementing
> > pidrefs.
> >
> >>> The question is - why does alloc_pidmap() use at least
> >>> two atomic ops and potentially loop to find a zero bit
> >>> in pidmap? Why not call alloc_pidmap() under pidmap_lock
> >>> and find zero pid in pidmap w/o any loops and atomics?
> >
> > Currently we search for zero bit lockless, why do you want
> > to do it under spin_lock ?
>
> Search isn't lockless. Look:
>
> while (1) {
>    if (!test_and_set_bit(...)) {
>        atomic_dec(&nr_free);
>        return pid;
>    }
>    ...
> }
>
> we use two atomic operations to find and set a bit in a map.

The finding of the zero bit is done without lock. (Search/lookup)

Then , the reservation of the found bit (test_and_set_bit) is done, and 
decrement of nr_free. It may fail because the search was done lockless.

Finding a zero bit in a 4096 bytes array may consume about 6000 cycles on 
modern hardware. Much more on SMP/NUMA machines, or on machines where 
PAGE_SIZE is 64K instead of 4K :)

You don't want to hold pidmad_lock for so long period.

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