Websites on PHP-FPM are not available and there is a php-fpm process that consumes 100% CPU on Plesk server

Plesk for Linux kb: technical ABT: Group B

Applicable to:

Plesk for Linux

Symptoms

Websites on Plesk using PHP-FPM handler hangs and might consume 100% CPU
A restart of the PHP-FPM service fixes the issue temporarily
The following error appears in the PHP-FPM error log (i.e. /var/log/plesk-php72-fpm/error.log):

CONFIG_TEXT: WARNING: [pool example.com] child 5023 said into stderr: "(5023): Error Cannot kill process 2349: Operation not permitted!"

The strace utility to the php-fpm process shows the following output:

# strace -p39053

strace: Process 39053 attached
kill(39005, SIGKILL) = -1 EPERM (Operation not permitted)
fcntl(4, F_GETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1, l_len=1, l_pid=39005}) = 0
kill(39005, SIGKILL) = -1 EPERM (Operation not permitted)
fcntl(4, F_GETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1, l_len=1, l_pid=39005}) = 0
kill(39005, SIGKILL) = -1 EPERM (Operation not permitted)
fcntl(4, F_GETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1, l_len=1, l_pid=39005}) = 0
kill(39005, SIGKILL) = -1 EPERM (Operation not permitted)

Such infinite loops arise on PHP below version 7.0 only

Cause

This is a PHP-FPM bug #74709.

Resolution

This bug was fixed for several PHP versions shipped with Plesk in Obsidian 18.0.34.
It is recommended to update Plesk to the latest release and switch PHP handlers to 7.4.16 or 8.0.3.

Note: As soon as a new version of 7.3 is released, it will get this fix in Plesk as well.

If the update is not possible for some reason or PHP by OS vendor is required, the following workarounds can be applied:

Disable OPCache module for the necessary PHP version:

Log in to Plesk.

Note: change the "X.X.X PHP handler" to the correct PHP version.
Go to Tools & Settings > PHP Settings > X.X.X PHP application
Uncheck the opcache checkbox and press the OK button.

If OPCache is necessary, switch the domains to another PHP handler, e.g. to FastCGI:

For many domains, it is recommended to manage PHP handlers via a service plan.

For locked subscriptions, changes have to be done per subscription.

All cases are described in the following article:

How to customize PHP settings in Plesk?

Forbid calling the "kill_all_lockers" function

Note: it is not possible to manage the disable_functions directive via the php.ini configuration file, because Plesk will overwrite them.

For a service plan:
1. Log in to Plesk
2. Go to Service Plans > Default > PHP Settings
  
  Note: change the "Default" service plan name in the path above to the correct one.
3. Add the following value at the end of the disable_functions field:
  
  CONFIG_TEXT: kill_all_lockers
4. Navigate to the Permissions tab and make sure that the Hosting settings management and Common PHP settings management permissions are not granted, otherwise, PHP settings will not be applied
5. Press the Update & Sync button
For subscription and for locked subscriptions:
1. Log in to Plesk
2. Go to Subscriptions > example.com > PHP Settings
3. Add the following value at the end of the disable_functions field:
  
  CONFIG_TEXT: kill_all_lockers
4. Click the OK button

Note: OPCache can be reset by the function opcache_reset(). For instance:

# /opt/plesk/php/7.2/bin/php -r "opcache_reset();";

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