Messages in this thread | | | Date | Tue, 17 Mar 2020 09:43:49 +0100 | From | Christian Brauner <> | Subject | Re: clone3: allow creation of time namespace with offset |
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On Tue, Mar 17, 2020 at 09:41:55AM +0100, Christian Brauner wrote: > On Tue, Mar 17, 2020 at 09:30:40AM +0100, Adrian Reber wrote: > > This is an attempt to add time namespace support to clone3(). I am not > > really sure which way clone3() should handle time namespaces. The time > > namespace through /proc cannot be used with clone3() because the offsets > > for the time namespace need to be written before a process has been > > created in that time namespace. This means it is necessary to somehow > > tell clone3() the offsets for the clocks. > > > > The time namespace offers the possibility to set offsets for > > CLOCK_MONOTONIC and CLOCK_BOOTTIME. My first approach was to extend > > 'struct clone_args` with '__aligned_u64 monotonic_offset' and > > '__aligned_u64 boottime_offset'. The problem with this approach was that > > it was not possible to set nanoseconds for the clocks in the time > > namespace. > > > > One of the motivations for clone3() with CLONE_NEWTIME was to enable > > CRIU to restore a process in a time namespace with the corresponding > > offsets. And although the nanosecond value can probably never be > > restored to the same value it had during checkpointing, because the > > clock keeps on running between CRIU pausing all processes and CRIU > > actually reading the value of the clocks, the nanosecond value is still > > necessary for CRIU to not restore a process where the clock jumps back > > due to CRIU restoring it with a nanonsecond value that is too small. > > > > Requiring nanoseconds as well as seconds for two clocks during clone3() > > means that it would require 4 additional members to 'struct clone_args': > > > > __aligned_u64 tls; > > __aligned_u64 set_tid; > > __aligned_u64 set_tid_size; > > + __aligned_u64 boottime_offset_seconds; > > + __aligned_u64 boottime_offset_nanoseconds; > > + __aligned_u64 monotonic_offset_seconds; > > + __aligned_u64 monotonic_offset_nanoseconds; > > }; > > > > To avoid four additional members to 'struct clone_args' this patchset > > uses another approach: > > > > __aligned_u64 tls; > > __aligned_u64 set_tid; > > __aligned_u64 set_tid_size; > > + __aligned_u64 timens_offset; > > + __aligned_u64 timens_offset_size; > > Hm, so for set_tid we did set_tid and set_tid_size which makes sense > because set_tid wasn't actually a struct. But I'm not a fan of > establishing a pattern whereby we always have to grow two member, the > object and it's size; at least when we're adding a struct. > So at a first glance here are two possible ideas: > - Don't add a size argument and assume that struct timens_offset won't > grow. I'm not sure how likely it is it will grow. > - Make the size the first member of struct timens_offset the size of the > struct. (See examples for this pattern in the sched syscalls.)
Oh, and I should point out right way that I consider this material for the v5.8 merge window. It's too late in this cycle to land this with any confidence in v5.7. Just so there's no disappointment. :) The good news is that this leaves us with ample time to figure this out.
Christian
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