Messages in this thread | | | From | Pedro Alves <> | Subject | Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids | Date | Fri, 25 Nov 2011 22:36:39 +0000 |
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On Friday 25 November 2011 17:03:26, Pavel Emelyanov wrote: > On 11/25/2011 08:54 PM, Oleg Nesterov wrote: > > How you can restore the multithread tracee? > > Don't know :) But if this approach sounds promising (I see, that now it's not, but...) I > can think more on it. > > > You need to unreserve/reserve the previous pid, and we have the same problems again, no? > > With the existing patch - yes, but as I said above - we need to decide which direction to > go and then I'll think further.
Thanks for thinking about all this. Being able to reserve pids would be nice, but I won't pretend to know the kernel's internals enough to be able to suggest a sane and acceptable way to do it. We'd have to be able to restore multi-threaded tracees (which would also mean that there are pids which leaders and others which are clones), and, we'd have to support a single-threaded tracer debugging (and spawning) more than one process, while not all tracees are involved in C/R. Maybe this (reservation) issue should be be considered an orthogonal mechanism for now.
> By now your opinion is to better stay where we are ;) but if moving is unavoidable, then > it's better to take the CLONE_CHILD_USEPIDS route. That's my position as well.
From the perspective of a client that is going to use this on a live system, CLONE_CHILD_USEPIDS seems a little better, in that the pid race is only against another task reusing the same pid, while with setting last_pid, you have a try/whoops-not-the-pid-I-want/kill/retry/rinse/repeat/ loop racing against all fork/clone's in the system, along with possibly needing to first to do a kill(PID, 0) to check whether the PID is available (unless setting last_pid already detects that).
BTW, it's not only GDB that would want this for live systems. Check out Berkeley Lab's C/R (https://ftg.lbl.gov/projects/CheckpointRestart/), where these guys use mixed kernel/userspace C/R in clusters for high-end scientific computing to e.g., migrate tasks between nodes, and pause/resume parallel MPI jobs (on live systems). (Apologies if everyone already knows about this :-) .)
From what I read from their papers, in their approach, from userspace, they spawn new children as usual, with whatever pids the kernel wants, and then afterwards (from userspace, but through a kernel module), magically change the process and threads's pids to the pids they really want. They also fixup the parent pids, and session ids after the fact, along the way.
-- Pedro Alves
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