[3.13.y-ckt stable] Patch "ARC: dw2 unwind: Remove falllback linear search thru FDE entries" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ARC: dw2 unwind: Remove falllback linear search thru FDE entries

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt32.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 417d11482bae6910f1d8ed20a71205929a09a190 Mon Sep 17 00:00:00 2001
From: Vineet Gupta <vgupta at synopsys.com>
Date: Mon, 23 Nov 2015 19:32:51 +0530
Subject: ARC: dw2 unwind: Remove falllback linear search thru FDE entries

commit 2e22502c080f27afeab5e6f11e618fb7bc7aea53 upstream.

Fixes STAR 9000953410: "perf callgraph profiling causing RCU stalls"

| perf record -g -c 15000 -e cycles /sbin/hackbench
|
| INFO: rcu_preempt self-detected stall on CPU
| 1: (1 GPs behind) idle=609/140000000000002/0 softirq=2914/2915 fqs=603
| Task dump for CPU 1:

in-kernel dwarf unwinder has a fast binary lookup and a fallback linear
search (which iterates thru each of ~11K entries) thus takes 2 orders of
magnitude longer (~3 million cycles vs. 2000). Routines written in hand
assembler lack dwarf info (as we don't support assembler CFI pseudo-ops
yet) fail the unwinder binary lookup, hit linear search, failing
nevertheless in the end.

However the linear search is pointless as binary lookup tables are created
from it in first place. It is impossible to have binary lookup fail while
succeed the linear search. It is pure waste of cycles thus removed by
this patch.

This manifested as RCU stalls / NMI watchdog splat when running
hackbench under perf with callgraph profiling. The triggering condition
was perf counter overflowing in routine lacking dwarf info (like memset)
leading to patheic 3 million cycle unwinder slow path and by the time it
returned new interrupts were already pending (Timer, IPI) and taken
rightaway. The original memset didn't make forward progress, system kept
accruing more interrupts and more unwinder delayes in a vicious feedback
loop, ultimately triggering the NMI diagnostic.

Signed-off-by: Vineet Gupta <vgupta at synopsys.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arc/kernel/unwind.c | 37 ++++---------------------------------
 1 file changed, 4 insertions(+), 33 deletions(-)

diff --git a/arch/arc/kernel/unwind.c b/arch/arc/kernel/unwind.c
index e550b11..2d6a36e 100644
--- a/arch/arc/kernel/unwind.c
+++ b/arch/arc/kernel/unwind.c
@@ -986,42 +986,13 @@ int arc_unwind(struct unwind_frame_info *frame)
 							    (const u8 *)(fde +
 									 1) +
 							    *fde, ptrType);
-				if (pc >= endLoc)
+				if (pc >= endLoc) {
 					fde = NULL;
-			} else
-				fde = NULL;
-		}
-		if (fde == NULL) {
-			for (fde = table->address, tableSize = table->size;
-			     cie = NULL, tableSize > sizeof(*fde)
-			     && tableSize - sizeof(*fde) >= *fde;
-			     tableSize -= sizeof(*fde) + *fde,
-			     fde += 1 + *fde / sizeof(*fde)) {
-				cie = cie_for_fde(fde, table);
-				if (cie == &bad_cie) {
 					cie = NULL;
-					break;
 				}
-				if (cie == NULL
-				    || cie == &not_fde
-				    || (ptrType = fde_pointer_type(cie)) < 0)
-					continue;
-				ptr = (const u8 *)(fde + 2);
-				startLoc = read_pointer(&ptr,
-							(const u8 *)(fde + 1) +
-							*fde, ptrType);
-				if (!startLoc)
-					continue;
-				if (!(ptrType & DW_EH_PE_indirect))
-					ptrType &=
-					    DW_EH_PE_FORM | DW_EH_PE_signed;
-				endLoc =
-				    startLoc + read_pointer(&ptr,
-							    (const u8 *)(fde +
-									 1) +
-							    *fde, ptrType);
-				if (pc >= startLoc && pc < endLoc)
-					break;
+			} else {
+				fde = NULL;
+				cie = NULL;
 			}
 		}
 	}
--
1.9.1