kpatch/kpatch: Optimize patch transition timing with a while loop

[oceansue]

Optimize the timing of patch transitions by using a while loop to check the transition status, replacing the for loop with a 1-second sleep. This approach reduces the likelihood of timeouts due to system load, potentially speeding up patch transitions.

[oceansue]

issues: #1409

[oceansue]

@joe-lawrence @jpoimboe PTAL. Thanks.
Fix SC2004 warnings in kpatch script.

[joe-lawrence]

Hi @oceansue , a few questions and comments about the PR:

• It removes the 1 second sleep from wait_for_patch_transition's loops around sysfs. As long as the transition isn't moving back and forth, I guess it might return the result slightly sooner (sub-second) at the expense of hammering on sysfs. Is that really a net win here? Or ...
• I don't understand how the recoding of the for (( i=0; i<POST_ENABLE_WAIT; i++ )); do loop iteration is supposed to work. Is it intended to lengthen the polling time? I could see that helping to detect a "long" transition instance.
• Code: I'm pretty sure that SECONDS isn't defined in the kpatch script. Missing code hunks?
• Minor nit about shellcheck: if the commit introduces new code, just squash it into the introducing commit (no need for a separate commit). If the commit is correcting existing code, it may make sense to spin out to its own commit.

[oceansue]

Hi @joe-lawrence ,Thank you for your response:

• The reason for removing the 1 second sleep was mentioned in the issues: kpatch load/unload patch transition has stalled #1409
• In Bash, SECONDS is a built-in special variable that tracks the number of seconds that have elapsed since the shell started.This variable increments automatically. So, your understanding of the for (( i=0; i<POST_ENABLE_WAIT; i++ )); do is correct. Changing for (( i=0; i<POST_ENABLE_WAIT; i++ )); do to while [ $i -lt $POST_SIGNAL_WAIT ]; do will not compromise functionality. It will exit the while loop only after the timeout is reached(after 15 seconds).
• I have already squashed the "Fix SC2004 warnings in kpatch script" commit into the previous commit.

I‘ve been considering whether there might be a better solution for this issue (#1409) on my system. But so far, This patch seems to be working well. However, It does result in a short-term increase in CPU load.

[joe-lawrence]

Hi @oceansue :

• Thanks for the explanation of the bash SECONDS variable -- that feature is new to me and looks like it could be handy to use!
• Now I understand now why the patch changed the loop construct as it needs to iterate many times and not just from 1 to N in whole number increments.
• That said, I still don't understand how the patch helps the case of a long patch transition (as noted in kpatch load/unload patch transition has stalled #1409, "the module transition time is over 1500 seconds"). As I understand the new code, both POST_ENABLE_WAIT and POST_SIGNAL_WAIT remain fixed at 15 and 60 seconds respectively. Only the polling frequency has increased (to as fast as possible). In that case, what is the difference if the code polls (and fails) once every 14 out of 15 seconds, or polls a million times for 14 seconds and then succeeds on the 15th second?

Given the long transition time reported in #1409, I might have expected a change like:

diff --git a/kpatch/kpatch b/kpatch/kpatch
index edfccfead917..0ca6aaf78fd1 100755
--- a/kpatch/kpatch
+++ b/kpatch/kpatch
@@ -26,9 +26,9 @@
 INSTALLDIR=/var/lib/kpatch
 SCRIPTDIR="$(readlink -f "$(dirname "$(type -p "$0")")")"
 VERSION="0.9.9"
-POST_ENABLE_WAIT=15    # seconds
-POST_SIGNAL_WAIT=60    # seconds
-MODULE_REF_WAIT=15     # seconds
+POST_ENABLE_WAIT="${POST_ENABLE_WAIT:-15}"     # seconds
+POST_SIGNAL_WAIT="${POST_SIGNAL_WAIT:-60}"     # seconds
+MODULE_REF_WAIT="${MODULE_REF_WAIT:-15}"       # seconds
 
 # How many times to try loading the patch if activeness safety check fails.
 MAX_LOAD_ATTEMPTS=5

which would let the user set POST_ENABLE_WAIT, POST_SIGNAL_WAIT, MODULE_REF_WAIT environment variables without the script overriding them (ie, $ POST_ENABLE_WAIT=1500 ./kpatch/kpatch).

[oceansue]

I've been trying to fix this recently, but unfortunately, I haven't made any progress. It seems that this commit is the only one that can solve the problem I'm encountering.
I want to find a way to minimize the modification of code functionality and meet my needs.

This is the patch I'm considering.

diff --git a/kpatch/kpatch b/kpatch/kpatch
index edfccfe..815dacb 100755
--- a/kpatch/kpatch
+++ b/kpatch/kpatch
@@ -245,29 +245,32 @@ signal_stalled_processes() {

 wait_for_patch_transition() {
        local module="$1"
-       local i
+       local i=0

        in_transition "$module" || return 0

        echo "waiting (up to $POST_ENABLE_WAIT seconds) for patch transition to complete..."
-       for (( i=0; i<POST_ENABLE_WAIT; i++ )); do
+       start_time=$SECONDS
+       while [ $i -lt $POST_ENABLE_WAIT ]; do
                if ! in_transition "$module" ; then
                        echo "transition complete ($i seconds)"
+                      sleep 1s
                        return 0
                fi
-               sleep 1s
+               i=$(( SECONDS - start_time ))
        done

        echo "patch transition has stalled!"
        signal_stalled_processes

        echo "waiting (up to $POST_SIGNAL_WAIT seconds) for patch transition to complete..."
-       for (( i=0; i<POST_SIGNAL_WAIT; i++ )); do
+       start_time=$SECONDS
+       while [ $i -lt $POST_SIGNAL_WAIT ]; do
                if ! in_transition "$module" ; then
                        echo "transition complete ($i seconds)"
                        return 0
                fi
-               sleep 1s
+               i=$(( SECONDS - start_time ))
        done

        return 1

[joe-lawrence]

@oceansue : with the minimal patch in place, what is reported by "transition complete ($i seconds)" ? Also, can you attach/inline an example kpatch that requires the modification? Thanks.

[oceansue]

Test environment:
kernel: 4.19.180
The cpu is the company's own 32 cores

use #1408 (comment) patch：

sudo ./kpatch load ../kpatch-build/livepatch-cmdline.ko 
loading patch module: ../kpatch-build/livepatch-cmdline.ko
waiting (up to 15 seconds) for patch transition to complete...
patch transition has stalled!
signaling stalled process(es):
waiting (up to 60 seconds) for patch transition to complete...
transition complete (11 seconds)
ocean@ocean-PC:~/kpatch/kpatch$ sudo ./kpatch unload ../kpatch-build/livepatch-cmdline.ko 
disabling patch module: livepatch_cmdline
waiting (up to 15 seconds) for patch transition to complete...
patch transition has stalled!
signaling stalled process(es):
waiting (up to 60 seconds) for patch transition to complete...
transition complete (8 seconds)
unloading patch module: livepatch_cmdline

revert #1408 (comment) patch：

ocean@ocean-PC:~/kpatch/kpatch$ sudo ./kpatch load ../kpatch-build/livepatch-cmdline.ko 
module already loaded, re-enabling
waiting (up to 15 seconds) for patch transition to complete...
patch transition has stalled!
signaling stalled process(es):
waiting (up to 60 seconds) for patch transition to complete...

Stalled processes:
module livepatch_cmdline did not complete its transition, disabling...
waiting (up to 15 seconds) for patch transition to complete...
transition complete (2 seconds)
kpatch: error: failed to re-enable module livepatch_cmdline (transition stalled), patch disabled

For other CPUs, if the transition can be completed in 15 seconds, the patch will have no effect. If it cannot be completed in more than 15 seconds, the program will enter a 60 second transition period during which the program will frequently access the sys interface to speed up the transition.

Also, I found that the bug seems to be caused by multiple cores, and when I limit the number of cores to less than 8, the transformation can be done successfully. When the number of cores is more than 8, the transformation doesn't go so well and requires this patch to fix it.I was unable to find a multi-core (>32) processor with x86 architecture to verify.

[joe-lawrence]

@jpoimboe - can you have a look at this report/patch, I think I'm just missing something on how/why it works.

[jpoimboe]

For other CPUs, if the transition can be completed in 15 seconds, the patch will have no effect.

Hm? Look closer, the patch actually removes the 1s sleep even for the first 15 seconds. Hammering the CPU with sysfs reads is not going to be an acceptable solution.

Is this with the standard cmdline-string.patch? If so, I suspect something is going wrong on the kernel side, possibly livepatch itself. Maybe the unwinder is encountering errors. Kernel 4.19 is ancient, who knows what it could be...

[oceansue]

@joe-lawrence @jpoimboe Thanks for the replies, the problem has been solved, according to the new commit from kernel.org, after applying it to the 4.19.0 kernel. Close the issue. Thanks again.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/livepatch/transition.c?h=v6.12-rc2&id=5de62ea84abd732ded7c5569426fd71c0420f83e