sysctl: make epp select proc serialized.

[weike.chen_dell.com]

Summary

On Dell Lati5440 laptop (RaptorLake U), 'sysctl -a' hang due to epp select.

Test Plan

'sysctl -a'

[zlei]

@weike.chen_dell.com
I'm running sysctl dev.hwpstate_intel | grep epp or sysctl -a and all looks sane.

I'm not understanding how CTLFLAG_NEEDGIANT solves the problem. Maybe we firstly need to know where and why sysctl_epp_select() got blocked infinitely.

[weike.chen_dell.com]

In D42993#982163, @zlei wrote:

@weike.chen_dell.com
I'm running sysctl dev.hwpstate_intel | grep epp or sysctl -a and all looks sane.

I'm not understanding how CTLFLAG_NEEDGIANT solves the problem. Maybe we firstly need to know where and why sysctl_epp_select() got blocked infinitely.

Were you trying it on RaptorLake U CPU? So far, for this is issue, we only reproduce it on RaptorLake U (Dell Latitude 5440). This could be some risk conditions since I check then original code is serialized the process.

[zlei]

In D42993#982531, @weike.chen_dell.com wrote:

In D42993#982163, @zlei wrote:

@weike.chen_dell.com
I'm running sysctl dev.hwpstate_intel | grep epp or sysctl -a and all looks sane.

I'm not understanding how CTLFLAG_NEEDGIANT solves the problem. Maybe we firstly need to know where and why sysctl_epp_select() got blocked infinitely.

Were you trying it on RaptorLake U CPU?

No, I tested on old I5 CPUs.

'sysctl -a' hang
So far, for this is issue, we only reproduce it on RaptorLake U (Dell Latitude 5440)

May you please file a problem report to https://bugs.freebsd.org/bugzilla/ ? Phabricator is really not good place to discussing bugs like this.

This could be some risk conditions since I check then original code is serialized the process.

Possibly true if it could not be repeated reliably.