On Mon, 3 Dec 2018 22:51:52 +0100
Arnd Bergmann [off-list ref] wrote:

On Mon, Dec 3, 2018 at 8:22 PM Will Deacon [off-list ref] wrote:

quoted

Hi Anders,

On Fri, Nov 30, 2018 at 04:09:56PM +0100, Anders Roxell wrote:  

quoted

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 <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <redacted>
---
 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

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();
+
 	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();
 }
 

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