Support setpriority

[Sergio Schvezov]

seccomp blocks setpriority making electron based apps unusable

[Jamie Strandboge (jdstrand)]

This requires https://bugs.launchpad.net/ubuntu/+source/ubuntu-core-launcher/+bug/1446748

[Kyle Fazzari (kyrofa)]

Or ERRNO instead of KILL. Think that's a long way off?

[Jamie Strandboge (jdstrand)]

ERRNO instead of KILL is a separate thing that will come but will require upstream kernel changes (and therefore also backports to our reference and vendor kernels).

[Evan (ev)]

This is also needed by Cassandra, which uses thread priorities to keep periodic tasks from taking precedence over its core workload:

https://github.com/evandandrea/cassandra-snap/commit/c47a44402ccb03abaecf4ae9474cd22c2661efd5

[Daniel Manrique (roadmr)]

I was able to circumvent this by patching the source code I was snapping to comment out one nice() call. This was feasible since it was just that one call, but may not be so easy for more complex software. This was done in the gnuchess snap (in the store), for the xboard part which tries to nice() the chess engine.

[Mark Shuttleworth (sabdfl)]

Good catch, seems reasonable and a very easy interface to add, patch
welcome!

Mark

[Jamie Strandboge (jdstrand)]

Snaps will be able to set their own priority to positive nice values by default once snap-confine >= 1.0.34 (already released upstream) is available in the various distros and snapd can start using the feature. AIUI, people are working to land this in Ubuntu now.

[Mark Shuttleworth (sabdfl)]

Ah, thats a nice outcome :)

[Jamie Strandboge (jdstrand)]

@Mark, on a related note, you mentioning adding an interface gave me the idea that we could add a 'process-control' (or something) interface that would allow, for example, htop to renice any process to any nice value as well as sending arbitrary signals to arbitrary processes. A powerful interface to be sure, but a useful one for a management snap. I've added a card to pursue this with the interfaces team.

[Mark Shuttleworth (sabdfl)]

Great thinking Jamie, thank you!

Mark

[Olivier Tilloy (osomon)]

This is also needed by oxide (web engine based on chromium).

[Jamie Strandboge (jdstrand)]

FYI, process-control interface is now merged in trunk (this is bug https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1598225).

[Evan (ev)]

Marking this as fixed. process-control has landed in a released version of snapd.

[Evan (ev)]

And thanks Jamie! It helped with Cassandra and we've had a recent surge in Electron apps where it's also proven useful :)

[Marco Trevisan (Treviño) (3v1n0)]

I've been trying to snap an electron app as per these changes, but I still see this setpriority call being blocked:

type=1326 audit(1477696911.141:2201): auid=4294967295 uid=1000 gid=1000 ses=4294967295 pid=32160 comm="electron" exe="/snap/electron-quick-start/x1/lib/node_modules/electron/dist/electron" sig=31 arch=c000003e syscall=141 compat=0 ip=0x7fe0610a74a7 code=0x0

And I don't see any change by using the 'process-control' plug, is there anything missing?

[Marco Trevisan (Treviño) (3v1n0)]

Oh, sorry... I noticed I had to connect the plug/slots manually with:

 - sudo snap connect electron-quick-start:process-control ubuntu-core:process-control

[Jamie Strandboge (jdstrand)]

FYI, while the addition of the 'process-control' interface fixed this for people, snapd 2.23 will have changes in the default template that will allow certain uses of setpriority to work without process-control (specifically, when using (setpriority(PRIO_PROCESS, 0, 0-19), which is a common use case). For the other uses, process-control is of course still available to use.