Re: [Lse-tech] scalability of signal delivery for Posix Threads

[Andi Kleen <ak@suse.de>]

> I think this is a more general issue.  Special casing one

It just cannot be done in the general case without slowing
down sigaction significantly. Or maybe it can, but nobody
has proposed a way to do it so far. 

It's difficult to design for machines where a simple spinlock
doesn't work properly anymore.

> piece of it is only going to make performance more surprising,
> something I think should be avoided if at all possible.

The special case in particular would be signals directed to a specific TID;
compared to signals load balanced over the thread group which needs
shared writable state. To simplify the fast path you could also make
more simplications: no queueing (otherwise you would need to duplicate
a lot of state to handle that into the task_struct) and probably
no SIGCHILD which is also full of special cases.

-And