Messages in this thread | | | From | Eric Dumazet <> | Subject | Re: [RFC] kernel/pid.c pid allocation wierdness | Date | Fri, 16 Mar 2007 12:37:47 +0100 |
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On Friday 16 March 2007 11:57, Pavel Emelianov wrote: > Oleg Nesterov wrote: > > On 03/14, Eric W. Biederman wrote: > >> Pavel Emelianov <xemul@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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