[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 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). > said (see above), by avoiding page_list_for_each() within the > locked region you already significantly reduce lock contention. > I.e. you need another means to find pages awaiting to be Right, but I don't have better ideas beside using page_list_for_each() or delist pages temporarily from heap list. > scrubbed. You may want to leverage that the allocation path > does the scrubbing if needed (e.g. by not scrubbing the first > of any set of contiguous free pages on the idle path, linking up > all other ones recognizing that their link fields are unused while > on an order-greater-than-zero free list). It sounds like another list have to be introduced, but I don't think this can help to get rid of lock contention or the similar race condition. -- Regards, -Bob _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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