[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 3/3] xen: use idle vcpus to scrub pages
>>> On 25.07.14 at 09:28, <bob.liu@xxxxxxxxxx> wrote: > On 07/25/2014 02:51 PM, Jan Beulich wrote: >>>>> On 25.07.14 at 02:42, <bob.liu@xxxxxxxxxx> wrote: >> >>> On 07/24/2014 02:24 PM, Jan Beulich wrote: >>>>>>> On 24.07.14 at 04:08, <bob.liu@xxxxxxxxxx> wrote: >>>> >>>>> On 07/23/2014 03:28 PM, Jan Beulich wrote: >>>>>>>>> On 15.07.14 at 11:16, <bob.liu@xxxxxxxxxx> wrote: >>>>>>> After so many days I haven't make a workable solution if don't remove >>>>>>> pages temporarily. The hardest part is iterating the heap free list >>>>>>> without holding heap_lock because if holding the lock it might be heavy >>>>>>> lock contention. >>>>>>> So do you think it's acceptable if fixed all other concerns about this >>>>>>> patch? >>>>>> >>>>>> No, I don't think so. Instead I'm of the opinion that you may have >>>>>> worked in the wrong direction: Rather than not taking the heap lock >>>>>> at all, it may also be sufficient to shrink the lock holding time (i.e. >>>>>> avoid long loops with the lock held). >>>>>> >>>>> >>>>> But I still think have to drop pages from heap list temporarily else >>>>> heap lock must be taken for a long time to get rid of E.g. below race >>>>> condition. >>>>> >>>>> A: alloc path B: idle loop >>>>> >>>>> spin_lock(&heap_lock) >>>>> page_list_for_each( pg, &heap(node, zone, order) ) >>>>> if _PGC_need_scrub is set, break; >>>>> spin_unlock(&heap_lock) >>>>> >>>>> if ( test_bit(_PGC_need_scrub, pg) >>>>> >>>>> ^^^^ >>>>> spin_lock(&heap_lock) >>>>> delist page >>>>> spin_unlock(&heap_lock) >>>>> >>>>> write data to this page >>>>> >>>>> scrub_one_page(pg) >>>>> ^^^ will clean useful data >>>> >>>> No (and I'm sure I said so before): The only problem is with the >>>> linked list itself; the page contents are not a problem - the >>>> allocation path can simply wait for the already suggested >>>> _PGC_scrubbing flag to clear before returning. And as already >>> >>> The page contents are a problem if the race condition I mentioned in >>> previous email happen. >>> >>> Because there is a time window between checking the PGC_need_scrub flag >>> and doing the real scrub in idle thread, the idle thread will still >>> scrub a page after that page have been allocated by allocation path and >>> been used(and have been written some useful data). >> >> Did you really read all of my previous reply? > > Sure, may be I misunderstood your reply. > If the allocation path can wait for the flag there is no problem, but I > remember you suggested to do the scrubbing also in allocation path in > which case I think this race condition will happen. Two CPUs scrubbing the same page simultaneously is not a correctness problem, only a performance one: The ultimate result is a scrubbed page, but there may be unnecessary bus contention. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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