On Tue, Oct 7, 2014 at 9:07 AM, PINTU KUMAR <pintu.k@xxxxxxxxxxx> wrote: > ----- Original Message ----- >> From: Colin Cross <ccross@xxxxxxxxxxx> >> To: pintu.k@xxxxxxxxxxx >> Cc: Laura Abbott <lauraa@xxxxxxxxxxxxxx>; Heesub Shin <heesub.shin@xxxxxxxxxxx>; "akpm@xxxxxxxxxxxxxxxxxxxx" <akpm@xxxxxxxxxxxxxxxxxxxx>; "gregkh@xxxxxxxxxxxxxxxxxxx" <gregkh@xxxxxxxxxxxxxxxxxxx>; "john.stultz@xxxxxxxxxx" <john.stultz@xxxxxxxxxx>; "rebecca@xxxxxxxxxxx" <rebecca@xxxxxxxxxxx>; "devel@xxxxxxxxxxxxxxxxxxxx" <devel@xxxxxxxxxxxxxxxxxxxx>; "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>; IQBAL SHAREEF <iqbal.ams@xxxxxxxxxxx>; "pintu_agarwal@xxxxxxxxx" <pintu_agarwal@xxxxxxxxx>; Vishnu Pratap Singh <vishnu.ps@xxxxxxxxxxx>; "cpgs@xxxxxxxxxxx" <cpgs@xxxxxxxxxxx> >> Sent: Monday, 6 October 2014 11:01 PM >> Subject: Re: Re: [PATCH 1/1] [ion]: system-heap use PAGE_ALLOC_COSTLY_ORDER for high order >> >> On Mon, Oct 6, 2014 at 9:26 AM, PINTU KUMAR <pintu.k@xxxxxxxxxxx> wrote: >> >>> >>> Hi, >>> >________________________________ >>> > From: Laura Abbott <lauraa@xxxxxxxxxxxxxx> >>> >To: Heesub Shin <heesub.shin@xxxxxxxxxxx>; Pintu Kumar >> <pintu.k@xxxxxxxxxxx>; akpm@xxxxxxxxxxxxxxxxxxxx; >> gregkh@xxxxxxxxxxxxxxxxxxx; john.stultz@xxxxxxxxxx; rebecca@xxxxxxxxxxx; >> ccross@xxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx >>> >Cc: iqbal.ams@xxxxxxxxxxx; pintu_agarwal@xxxxxxxxx; >> vishnu.ps@xxxxxxxxxxx >>> >Sent: Monday, 6 October 2014 7:37 PM >>> >Subject: Re: [PATCH 1/1] [ion]: system-heap use PAGE_ALLOC_COSTLY_ORDER >> for high order >>> > >>> > >>> >On 10/6/2014 3:27 AM, Heesub Shin wrote: >>> > >>> > >>> > >>> > >>> >> Hello Kumar, >>> >> >>> >> On 10/06/2014 05:31 PM, Pintu Kumar wrote: >>> >>> The Android ion_system_heap uses allocation fallback mechanism >>> >>> based on 8,4,0 order pages available in the system. >>> >>> It changes gfp flags based on higher order allocation request. >>> >>> This higher order value is hard-coded as 4, instead of using >>> >>> the system defined higher order value. >>> >>> Thus replacing this hard-coded value with >> PAGE_ALLOC_COSTLY_ORDER >>> >>> which is defined as 3. >>> >>> This will help mapping the higher order request in system heap >> with >>> >>> the actual allocation request. >>> >> >>> >> Quite reasonable. >>> >> >>> >> Reviewed-by: Heesub Shin <heesub.shin@xxxxxxxxxxx> >>> >> >>> >> BTW, Anyone knows how the allocation order (8,4 and 0) was >> decided? I >>> >> think only Google guys might know the answer. >>> >> >>> >> regards, >>> >> heesub >>> >> >>> > >>> >My understanding was this was completely unrelated to the costly order >>> >and was related to the page sizes corresponding to IOMMU page sizes >>> >(1MB, 64K, 4K). This won't make a difference for the uncached page >>> >pool case but for the not page pool case, I'm not sure if there >> would >>> >be a benefit for trying to get 32K pages with some effort vs. just >>> >going back to 4K pages. >>> >>> No, it is not just related to IOMMU case. It comes into picture also for >>> normal system-heap allocation (without iommu cases). >>> Also, it is applicable for both uncached and page_pool cases. >>> Please also check the changes under ion_system_heap_create. >>> Here the gfp_flags are set under the pool structure. >>> This value is used in ion_page_pool_alloc_pages. >>> In both the cases, it internally calls alloc_pages, with this gfp_flags. >>> Now, during memory pressure scenario, when alloc_pages moves to slowpath >>> this gfp_flags will be used to decide allocation retry. >>> In the current code, the higher-order flag is set only when order is >> greater than 4. >>> But, in MM, the order 4 is also considered as higher-order request. >>> This higher-order is decided based on PAGE_ALLOC_COSTLY_ORDER (3) value. >>> Hence, I think this value should be in sync with the MM code. >>> > >>> >Do you have any data/metrics that show a benefit from this patch? >>> I think it is not related to any data or metrics. >>> It is about replacing the hard-coded higher-order check to be in sync with >>> the MM code. >>> >> >> The selection of the orders used for allocation (8, then 4, then 0) is >> designed to match with the sizes often found in IOMMUs, but this isn't >> changing the order of the allocation, it is changing the GFP flags >> used for the order 4 allocation. Right now we are using the >> low_order_gfp_flags for order 4, this patch would change it to use >> high_order_gfp_flags. We originally used low_order_gfp_flags here >> because the MM subsystem can usually satisfy these allocations, and >> the additional load placed on the MM subsystem to kick off kswapd to >> free up more order 4 chunks is generally worth it. Using order 4 >> pages instead of order 0 pages can significantly improve the >> performance of many IOMMUs by reducing TLB pressure and time spent >> updating page tables. Unless you have data showing that this improves >> something, and doesn't just cause all allocations to be order 0 when >> under memory pressure, I don't suggest merging this. >> > > Ok agree. It is worth retrying the allocation with order-4 pages. > But, since 4 is considered higher order for MM and is greater than PAGE_ALLOC_COSTLY_ORDER. > I guess the retrying will not happen, because of the following check in page_alloc: > if (order > PAGE_ALLOC_COSTLY_ORDER) > goto nopage; It's not the retry that I'd be particularly worried about, it's kicking off kswapd, which will run in the background to replenish the pool of free order 4 pages. However, when c654345924f7cce87bb221b89db91cba890421ba (mm: remove __GFP_NO_KSWAPD) was merged (and then reverted, and then merged again), the __GFP_NO_KSWAPD flag was removed from high_order_gfp_flags, and when it was finally reverted again we did not re-add the flag, so we lost that behavior long ago. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
