Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

[Mathieu Desnoyers]

On 5/30/23 04:20, Florian Weimer wrote:
> * Mathieu Desnoyers:
>
> > > I don't see why we can't stick this directly into struct rseq because
> > > it's all public anyway.
> >
> > The motivation for moving this to a different cache line is to handle
> > the prior comment from Boqun, who is concerned that busy-waiting
> > repeatedly loading a field from struct rseq will cause false-sharing
> > and make other stores to that cache line slower, especially stores to
> > rseq_cs to begin rseq critical sections, thus slightly increasing the
> > overhead of rseq critical sections taken while mutexes are held.
>
> Hmm.  For context, in glibc, we have to place struct rseq on a fixed
> offset from the start of a page (or even some larger alignment) for all
> threads.  In the future (once we move the thread control block off the
> top of the userspace stack, where it resides since the LinuxThreads
> days), it is likely that the pointer difference between different
> threads will also be a multiple of a fairly large power of two
> (something like 2**20 might be common).  Maybe this will make caching
> even more difficult?
>
> > If we want to embed this field into struct rseq with its own cache
> > line, then we need to add a lot of padding, which is inconvenient.
> >
> > That being said, perhaps this is premature optimization, what do you
> > think ?
>
> Maybe?  I don't know how the access patterns will look like.  But I
> suspect that once we hit this case, performance will not be great
> anyway, so the optimization is perhaps unnecessary?

What I dislike though is that contention for any lock which busy-waits 
on the rseq sched_state would slow down all rseq critical sections of 
that thread, which is a side-effect we want to avoid.

I've done some more additional benchmarks on my 8-core AMD laptop, and I 
notice that things get especially bad whenever the store to 
rseq_abi->rseq_cs is surrounded by other instructions that need to be 
ordered with that store, e.g. a for loop doing 10 stores to a local 
variables. If it's surrounded by instructions that don't need to be 
ordered wrt that store (e.g. a for loop of 10 iterations issuing 
barrier() "memory" asm clobbers), then the overhead cannot be noticed 
anymore.

> The challenge is that once we put stuff at fixed offsets, we can't
> transparently fix it later.  It would need more auxv entries with
> further offsets, or accessing this data through some indirection,
> perhaps via vDSO helpers.

Perhaps this is more flexibility/complexity than we really need. One 
possible approach would be to split struct rseq into sub-structures, e.g.:

rseq_len = overall size of all sub-structures.
auxv AT_RSEQ_ALIGN = 256

auxv AT_RSEQ_FEATURE_SIZE = size of first portion of struct rseq,
                            at most 256 bytes, meant to contain fields
                            stored/loaded from the thread doing the
                            registration.
auxv AT_RSEQ_SHARED_FEATURE_SIZE =
                            size of 2nd portion of struct rseq,
                            starts at offset 256, at most 256 bytes,
                            meant to contain fields stored/loaded by
                            any thread.

Then we have this layout:

struct rseq {
  struct rseq_local {
    /* Fields accessed from local thread. */

  } __attribute__((aligned((256));
  struct rseq_shared {
    /* Shared fields. */

  } __attribute__((aligned(256));
} __attribute__((aligned(256));

And if someday AT_RSEQ_FEATURE_SIZE needs to grow over 256 bytes
(32 * u64), we can still extend with a new auxv entry after the "shared"
features.


> > > The TID field would be useful in its own right.
> >
> > Indeed, good point.
> >
> > While we are there, I wonder if we should use the thread_pointer() as
> > lock identifier, or if the address of struct rseq is fine ?
>
> Hard to tell until we'll see what the futex integration looks like, I
> think.

Good point. I can choose one way or another for the prototype, and then 
we'll see how things go with futex integration.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com