On Mon, Jul 27, 2020 at 03:31:16PM +0800, Yu Xu wrote: > On 7/25/20 4:22 AM, Linus Torvalds wrote: > > On Fri, Jul 24, 2020 at 12:27 PM Linus Torvalds > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > It *may* make sense to say "ok, don't bother flushing the TLB if this > > > is a retry, because we already did that originally". MAYBE. [...] > > We could say that we never need it at all for FAULT_FLAG_RETRY. That > > makes a lot of sense to me. > > > > So a patch that does something like the appended (intentionally > > whitespace-damaged) seems sensible. > > I tested your patch on our aarch64 box, with 128 online CPUs. [...] > There are two points to sum up. > > 1) the performance of page_fault3_process is restored, while the performance > of page_fault3_thread is about ~80% of the vanilla, except the case of 128 > threads. > > 2) in the case of 128 threads, test worker threads seem to get stuck, making > no progress in the iterations of mmap-write-munmap until a period of time > later. the test result is 0 because only first 16 samples are counted, and > they are all 0. This situation is easy to re-produce with large number of > threads (not necessarily 128), and the stack of one stuck thread is shown > below. > > [<0>] __switch_to+0xdc/0x150 > [<0>] wb_wait_for_completion+0x84/0xb0 > [<0>] __writeback_inodes_sb_nr+0x9c/0xe8 > [<0>] try_to_writeback_inodes_sb+0x6c/0x88 > [<0>] ext4_nonda_switch+0x90/0x98 [ext4] > [<0>] ext4_page_mkwrite+0x248/0x4c0 [ext4] > [<0>] do_page_mkwrite+0x4c/0x100 > [<0>] do_fault+0x2ac/0x3e0 > [<0>] handle_pte_fault+0xb4/0x258 > [<0>] __handle_mm_fault+0x1d8/0x3a8 > [<0>] handle_mm_fault+0x104/0x1d0 > [<0>] do_page_fault+0x16c/0x490 > [<0>] do_translation_fault+0x60/0x68 > [<0>] do_mem_abort+0x58/0x100 > [<0>] el0_da+0x24/0x28 > [<0>] 0xffffffffffffffff > > It seems quite normal, right? and I've run out of ideas. If threads get stuck here, it could be a stale TLB entry that's not flushed with Linus' patch. Since that's a write fault, I think it hits the FAULT_FLAG_TRIED case. Could you give my patch here a try as an alternative: https://lore.kernel.org/linux-mm/20200725155841.GA14490@gaia/ It leaves the spurious flush in place but only local (though note that in a guest under KVM, all local TLBIs are upgraded to inner-shareable, so you'd not get the performance benefit). -- Catalin
