Hi Ben: On 07/24/2011 07:55 PM, Benjamin Herrenschmidt wrote: > On Sun, 2011-07-24 at 11:18 -0600, David Ahern wrote: >> On 07/20/2011 03:57 PM, David Ahern wrote: >>> I am hoping someone familiar with PPC can help understand a panic that >>> is generated when capturing callchains with context switch events. >>> >>> Call trace is below. The short of it is that walking the callchain >>> generates a page fault. To handle the page fault the mmap_sem is needed, >>> but it is currently held by setup_arg_pages. setup_arg_pages calls >>> shift_arg_pages with the mmap_sem held. shift_arg_pages then calls >>> move_page_tables which has a cond_resched at the top of its for loop. If >>> the cond_resched() is removed from move_page_tables everything works >>> beautifully - no panics. >>> >>> So, the question: is it normal for walking the stack to trigger a page >>> fault on PPC? The panic is not seen on x86 based systems. >> >> Can anyone confirm whether page faults while walking the stack are >> normal for PPC? We really want to use the context switch event with >> callchains and need to understand whether this behavior is normal. Of >> course if it is normal, a way to address the problem without a panic >> will be needed. > > Now that leads to interesting discoveries :-) Becky, can you read all > the way and let me know what you think ? > > So, trying to walk the user stack directly will potentially cause page > faults if it's done by direct access. So if you're going to do it in a > spot where you can't afford it, you need to pagefault_disable() I > suppose. I think the problem with our existing code is that it's missing > those around __get_user_inatomic(). > > In fact, arguably, we don't want the hash code from modifying the hash > either (or even hashing things in). Our 64-bit code handles it today in > perf_callchain.c in a way that involves pretty much duplicating the > functionality of __get_user_pages_fast() as used by x86 (see below), but > as a fallback from a direct access which misses the pagefault_disable() > as well. > > I think it comes from an old assumption that this would always be called > from an nmi, and the explicit tracepoints broke that assumption. > > In fact we probably want to bump the NMI count, not just the IRQ count > as pagefault_disable() does, to make sure we prevent hashing. > > x86 does things differently, using __get_user_pages_fast() (a variant of > get_user_page_fast() that doesn't fallback to normal get_user_pages()). > > Now, we could do the same (use __gup_fast too), but I can see a > potential issue with ppc 32-bit platforms that have 64-bit PTEs, since > we could end up GUP'ing in the middle of the two accesses. > > Becky: I think gup_fast is generally broken on 32-bit with 64-bit PTE > because of that, the problem isn't specific to perf backtraces, I'll > propose a solution further down. > > Now, on x86, there is a similar problem with PAE, which is handled by > > - having gup disable IRQs > - rely on the fact that to change from a valid value to another valid > value, the PTE will first get invalidated, which requires an IPI > and thus will be blocked by our interrupts being off > > We do the first part, but the second part will break if we use HW TLB > invalidation broadcast (yet another reason why those are bad, I think I > will write a blog entry about it one of these days). > > I think we can work around this while keeping our broadcast TLB > invalidations by having the invalidation code also increment a global > generation count (using the existing lock used by the invalidation code, > all 32-bit platforms have such a lock). > > From there, gup_fast can be changed to, with proper ordering, check the > generation count around the loading of the PTE and loop if it has > changed, kind-of a seqlock. > > We also need the NMI count bump if we are going to try to keep the > attempt at doing a direct access first for perfs. > > Becky, do you feel like giving that a shot or should I find another > victim ? (Or even do it myself ... ) :-)
Did you have something in mind besides the patch Anton sent? We'll give that one a try and see how it works. (Thanks, Anton!) David > > Cheers, > Ben. > >> Thanks, >> David >> >>> >>> [<b0180e00>]rb_erase+0x1b4/0x3e8 >>> [<b00430f4>]__dequeue_entity+0x50/0xe8 >>> [<b0043304>]set_next_entity+0x178/0x1bc >>> [<b0043440>]pick_next_task_fair+0xb0/0x118 >>> [<b02ada80>]schedule+0x500/0x614 >>> [<b02afaa8>]rwsem_down_failed_common+0xf0/0x264 >>> [<b02afca0>]rwsem_down_read_failed+0x34/0x54 >>> [<b02aed4c>]down_read+0x3c/0x54 >>> [<b0023b58>]do_page_fault+0x114/0x5e8 >>> [<b001e350>]handle_page_fault+0xc/0x80 >>> [<b0022dec>]perf_callchain+0x224/0x31c >>> [<b009ba70>]perf_prepare_sample+0x240/0x2fc >>> [<b009d760>]__perf_event_overflow+0x280/0x398 >>> [<b009d914>]perf_swevent_overflow+0x9c/0x10c >>> [<b009db54>]perf_swevent_ctx_event+0x1d0/0x230 >>> [<b009dc38>]do_perf_sw_event+0x84/0xe4 >>> [<b009dde8>]perf_sw_event_context_switch+0x150/0x1b4 >>> [<b009de90>]perf_event_task_sched_out+0x44/0x2d4 >>> [<b02ad840>]schedule+0x2c0/0x614 >>> [<b0047dc0>]__cond_resched+0x34/0x90 >>> [<b02adcc8>]_cond_resched+0x4c/0x68 >>> [<b00bccf8>]move_page_tables+0xb0/0x418 >>> [<b00d7ee0>]setup_arg_pages+0x184/0x2a0 >>> [<b0110914>]load_elf_binary+0x394/0x1208 >>> [<b00d6e28>]search_binary_handler+0xe0/0x2c4 >>> [<b00d834c>]do_execve+0x1bc/0x268 >>> [<b0015394>]sys_execve+0x84/0xc8 >>> [<b001df10>]ret_from_syscall+0x0/0x3c >>> >>> Thanks, >>> David >> _______________________________________________ >> Linuxppc-dev mailing list >> Linuxppc-dev@lists.ozlabs.org >> https://lists.ozlabs.org/listinfo/linuxppc-dev > > _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev