Hi Minchan, On Friday, August 17, 2012 3:06 AM Minchan Kim wrote:
> On Thu, Aug 16, 2012 at 02:58:18PM +0100, Mel Gorman wrote: > > On Tue, Aug 14, 2012 at 05:57:06PM +0900, Minchan Kim wrote: > > > Now cma reclaims too many pages by __reclaim_pages which says > > > following as > > > > > > * Reclaim enough pages to make sure that contiguous allocation > > > * will not starve the system. > > > > > > Starve? What does it starve the system? The function which allocate > > > free page for migration target would wake up kswapd and do direct reclaim > > > if needed during migration so system doesn't starve. > > > > > > > I thought this patch was overkill at the time it was introduced but > > didn't have a concrete reason to reject it when I commented on it > > https://lkml.org/lkml/2012/1/30/136 . Marek did want this and followed > > up with "contiguous allocations should have higher priority than others" > > which I took to mean that he was also ok with excessive reclaim. > > I think OOM kill to background applications is more appropriate than > big latency of foreground(ex, Camera app) application in your mobile phone. > In other words, excessive reclaim is *really* bad which elapsed 8sec > in my test as worst case. :( > > > > > > Let remove __reclaim_pages and related function and fields. > > > > > > > That should be one patch and I don't object to it being removed as such > > but it's Marek's call. > > Marek. Any thought? Well, I've introduced this function as a result of solving Mel's request: http://www.spinics.net/lists/linux-mm/msg28485.html Before that I solved it almost the same as in your current patch. I'm aware of the fact that __reclaim_pages() approach might be a little overkill, but I didn't find anything better yet. > > > I modified split_free_page slightly because I removed __reclaim_pages, > > > isolate_freepages_range can fail by split_free_page's watermark check. > > > It's very critical in CMA because it ends up failing alloc_contig_range. > > > > > > > This is a big change and should have been in a patch on its > > own. split_free_page checks watermarks because if the watermarks are > > not obeyed a zone can become fully allocated. This can cause a system to > > livelock under certain circumstances if a page cannot be allocated and a > > free page is required before other pages can be freed. > > > > > I think we don't need the check in case of CMA because CMA allocates > > > free pages by alloc_pages, not isolate_freepages_block in migrate_pages > > > so watermark is already checked in alloc_pages. > > > > It uses alloc_pages when migrating pages out of the CMA area but note > > that it uses isolate_freepages_block when allocating the CMA buffer when > > alloc_contig_range calls isolate_freepages_range > > > > isolate_freepages_range(unsigned long start_pfn, unsigned long end_pfn) > > { > > for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) { > > isolated = isolate_freepages_block(pfn, block_end_pfn, > > &freelist, true); > > } > > map_pages(&freelist); > > } > > > > so the actual CMA allocation itself is not using alloc_pages. By removing > > the watermark check you allow the CMA to breach watermarks and puts the > > system at risk of livelock. > > Fair enough. I will look into that. We will also looks into this issue. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
