Messages in this thread | | | Date | Thu, 11 May 2023 12:12:32 +0800 | Subject | Re: [RFD] posix-timers: CRIU woes | From | Pavel Tikhomirov <> |
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On 10.05.2023 16:30, Thomas Gleixner wrote: > Pavel! > > On Wed, May 10 2023 at 12:36, Pavel Tikhomirov wrote: >> On 10.05.2023 05:42, Thomas Gleixner wrote: >>> So because of that half thought out user space ABI we are now up the >>> regression creek without a paddle, unless CRIU can accomodate to a >>> different restore mechanism to lift this restriction from the kernel. >>> >>> Thoughts? >> >> Maybe we can do something similar to /proc/sys/kernel/ns_last_pid? >> Switch to per-(process->signal) idr based approach with idr_set_cursor >> to set next id for next posix timer from new sysctl? > > I'm not a fan of such sysctls. We have already too many of them and that > particular one does not buy much.
Sorry, it was a bad idea, what you suggest below is much better.
> > We can simply let timer_create() or a new syscall create a timer at a > given ID.
Yes this would work for CRIU. (note: in neighbor thread Andrei writes about adding a bit to sigevent.sigev_notify to request a timer with a specified id, new syscall is also a good option)
> > That allows CRIU to restore any checkpointed process no matter which > kernel version it came from without doing this insane create/delete > dance.
Yes, for CRIU this kind of API change is a big improvement.
> > The downside is that this allows to create stupidly sparse timer IDs > even for the non CRIU case, which increases per process kernel memory > consumption and creates slightly more overhead in the signal delivery > path. The latter is a burden on the process owning the timer and not > affecting expiry, which is a context stealing operation. The memory part > needs eventually some thoughts vs. accounting. > > If the 'explicit at ID' option is not used then the ID mechanism is > optimzied for dense IDs by using the first available ID in a bottom up > search, which recovers holes created by a timer_delete() operation.
Not sure how kernel memory consumption increases with sparse timer IDs, global hashtable (posix_timers_hashtable) is the same size anyway, entries in hlists can be distributed differently as hash depends on id directly but we have same number of entries. Probably I miss something, why do we need dense IDs?
> > Thanks, > > tglx
-- Best regards, Tikhomirov Pavel Senior Software Developer, Virtuozzo.
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