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File crash-task.c-avoid-unnecessary-cpu-cycles-in-stkptr_to_tas.patch of Package crash (Revision a874117b9fcdd1f71e4e21ae0ae0b469)

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From 46cfe1f5aed3b1950df505d71553c13abab060a6 Mon Sep 17 00:00:00 2001
From: Hari Bathini <hbathini@linux.ibm.com>
Date: Sat, 14 Nov 2020 02:16:30 +0530
Subject: [PATCH] task.c: avoid unnecessary cpu cycles in stkptr_to_task()

References: bsc#1178827 ltc#189279
Upstream: 7.2.9
Git-commit: 46cfe1f5aed3b1950df505d71553c13abab060a6

While stkptr_to_task does the job of trying to match a stack pointer
to a task, it runs through each task's stack to find whether the given
SP falls into its range. This can be a very expensive operation, if
the vmcore is from a system running too many tasks. It can get even
worse when the total number of CPUs on the system is in the order of
thousands. Given the expensive nature of the operation, it must be
optimized as much as possible. Possible options to optimize:

1) Get min & max of the stack range in first pass and use these
     values against the given SP to decide whether or not to proceed
     with stack lookup.
  2) Use multithreading to parallely update irq_tasks.
  3) Skip stkptr_to_task() when SP is 0

Though option 3 is a low hanging fruit, it significantly improved the
time taken between starting crash utility & reaching crash prompt.
Implement option 3 to optimize while listing the other two options
as TODO items for follow-up.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
---
 task.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/task.c b/task.c
index 8dd2b96d83fd..423cd454502b 100644
--- a/task.c
+++ b/task.c
@@ -713,6 +713,7 @@ irqstacks_init(void)
 	} else 
 		error(WARNING, "cannot determine hardirq_ctx addresses\n");
 
+	/* TODO: Use multithreading to parallely update irq_tasks. */
 	for (i = 0; i < NR_CPUS; i++) {
 		if (!(tt->hardirq_ctx[i]))
 			continue;
@@ -5005,6 +5006,10 @@ pid_exists(ulong pid)
 /*
  *  Translate a stack pointer to a task, dealing with possible split.
  *  If that doesn't work, check the hardirq_stack and softirq_stack.
+ *
+ * TODO: This function can be optimized by getting min & max of the
+ *       stack range in first pass and use these values against the
+ *       given SP to decide whether or not to proceed with stack lookup.
  */
 ulong
 stkptr_to_task(ulong sp)
@@ -5013,6 +5018,9 @@ stkptr_to_task(ulong sp)
         struct task_context *tc;
 	struct bt_info bt_info, *bt;
 
+	if (!sp)
+		return NO_TASK;
+
 	bt = &bt_info;
         tc = FIRST_CONTEXT();
         for (i = 0; i < RUNNING_TASKS(); i++, tc++) {
-- 
2.26.2

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