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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Steve, Arnd, On Tue, Dec 04, 2018 at 12:50:12AM -0500, Steven Rostedt wrote: > On Mon, 3 Dec 2018 22:51:52 +0100 > Arnd Bergmann wrote: > > On Mon, Dec 3, 2018 at 8:22 PM Will Deacon wrote: > > > On Fri, Nov 30, 2018 at 04:09:56PM +0100, Anders Roxell wrote: > > > > Both of those functions end up calling ftrace_modify_code(), which is > > > > expensive because it changes the page tables and flush caches. > > > > Microseconds add up because this is called in a loop for each dyn_ftrace > > > > record, and this triggers the softlockup watchdog unless we let it sleep > > > > occasionally. > > > > Rework so that we call cond_resched() before going into the > > > > ftrace_modify_code() function. > > > > > > > > Co-developed-by: Arnd Bergmann > > > > Signed-off-by: Arnd Bergmann > > > > Signed-off-by: Anders Roxell > > > > --- > > > > arch/arm64/kernel/ftrace.c | 10 ++++++++++ > > > > 1 file changed, 10 insertions(+) > > > > > > It sounds like you're running into issues with the existing code, but I'd > > > like to understand a bit more about exactly what you're seeing. Which part > > > of the ftrace patching is proving to be expensive? > > > > > > The page table manipulation only happens once per module when using PLTs, > > > and the cache maintenance is just a single line per patch site without an > > > IPI. > > > > > > Is it the loop in ftrace_replace_code() that is causing the hassle? > > > > Yes: with an allmodconfig kernel, the ftrace selftest calls ftrace_replace_code > > to look >40000 through ftrace_make_call/ftrace_make_nop, and these > > end up calling Ok, 40000 invocations would do it! > > static int __kprobes __aarch64_insn_write(void *addr, __le32 insn) > > { > > void *waddr = addr; > > unsigned long flags = 0; > > int ret; > > > > raw_spin_lock_irqsave(&patch_lock, flags); > > waddr = patch_map(addr, FIX_TEXT_POKE0); > > > > ret = probe_kernel_write(waddr, &insn, AARCH64_INSN_SIZE); > > > > patch_unmap(FIX_TEXT_POKE0); > > raw_spin_unlock_irqrestore(&patch_lock, flags); > > > > return ret; > > } > > int __kprobes aarch64_insn_patch_text_nosync(void *addr, u32 insn) > > { > > u32 *tp = addr; > > int ret; > > > > /* A64 instructions must be word aligned */ > > if ((uintptr_t)tp & 0x3) > > return -EINVAL; > > > > ret = aarch64_insn_write(tp, insn); > > if (ret == 0) > > __flush_icache_range((uintptr_t)tp, > > (uintptr_t)tp + AARCH64_INSN_SIZE); > > > > return ret; > > } > > > > which seems to be where the main cost is. This is with inside of > > qemu, and with lots of debugging options (in particular > > kcov and ubsan) enabled, that make each function call > > more expensive. > > I was thinking more about this. Would something like this work? > > -- Steve > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > index 8ef9fc226037..42e89397778b 100644 > --- a/kernel/trace/ftrace.c > +++ b/kernel/trace/ftrace.c > @@ -2393,11 +2393,14 @@ void __weak ftrace_replace_code(int enable) > { > struct dyn_ftrace *rec; > struct ftrace_page *pg; > + bool schedulable; > int failed; > > if (unlikely(ftrace_disabled)) > return; > > + schedulable = !irqs_disabled() & !preempt_count(); Looks suspiciously like a bitwise preemptible() to me! > + > do_for_each_ftrace_rec(pg, rec) { > > if (rec->flags & FTRACE_FL_DISABLED) > @@ -2409,6 +2412,8 @@ void __weak ftrace_replace_code(int enable) > /* Stop processing */ > return; > } > + if (schedulable) > + cond_resched(); > } while_for_each_ftrace_rec(); > } If this solves the problem in core code, them I'm all for it. Otherwise, I was thinking of rolling our own ftrace_replace_code() for arm64, but that's going to involve a fair amount of duplication. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel