Re: [RFC PATCH] LTTng relay buffer allocation, read, write

[Peter Zijlstra]

On Mon, 2008-09-29 at 11:50 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> > On Sat, 2008-09-27 at 09:40 -0400, Mathieu Desnoyers wrote:
> > > As I told Martin, I was thinking about taking an axe and moving stuff around in
> > > relay. Which I just did.
> > > 
> > > This patch reimplements relay with a linked list of pages. Provides read/write
> > > wrappers which should be used to read or write from the buffers. It's the core
> > > of a layered approach to the design requirements expressed by Martin and
> > > discussed earlier.
> > > 
> > > It does not provide _any_ sort of locking on buffer data. Locking should be done
> > > by the caller. Given that we might think of very lightweight locking schemes, it
> > > makes sense to me that the underlying buffering infrastructure supports event
> > > records larger than 1 page.
> > > 
> > > A cache saving 3 pointers is used to keep track of current page used for the
> > > buffer for write, read and current subbuffer header pointer lookup. The offset
> > > of each page within the buffer is saved in the page frame structure to permit
> > > cache lookup without extra locking.
> > > 
> > > TODO : Currently, no splice file operations are implemented. Should come soon.
> > > The idea is to splice the buffers directly into files or to the network.
> > > 
> > > This patch is released as early RFC. It builds, that's about it. Testing will
> > > come when I implement the splice ops.
> > 
> > Why? What aspects of Steve's ring-buffer interface will hinder us
> > optimizing the implementation to be as light-weight as you like?
> > 
> > The thing is, I'd like to see it that light as well ;-)
> > 
> 
> Ok, I'll try to explain my point of view. The thing is : I want those
> binary buffers to be exported to userspace, and I fear that the approach
> taken by Steven (let's write "simple" C structure directly into the
> buffers) will in fact be much more _complex_ (due to subtle compiler
> dependencies) that doing our own event payload (event data) format.

The only compiler dependant thing in there is the bitfield, which could
be recoded using regular bitops.

I'm not seeing anything particularly worrysome about that.

> Also, things such as 
> ring_buffer_lock: A way to lock the entire buffer.
> ring_buffer_unlock: unlock the buffer.
> will probably become a problem when trying to go for a more efficient
> locking scheme.

You can do

stop:
cpu_buffer->stop=1;
smp_wmb();
sched_sync();

start:
smp_wmb();
cpu_buffer->stop=0;


write:
if (unlikely(smp_rmb(), cpu_buffer->stop))) {
  return -EBUSY;
    or
  while (cpu_buffer->stop)
    cpu_relax();
}

The read in the fast path is just a read, nothing fancy...

> ring_buffer_peek: Look at a next item in the cpu buffer.
> This kind of feature is useless for data extraction to userspace and
> poses serious synchronization concerns if you have other writers in the
> same buffer.

Sure, its probably possible to rework the merge-iterator to use consume,
but that would require it having storage for 1 event, which might be a
bit messy.

How would your locking deal with this? - it really is a requirement to
be able to merge-sort-iterate the output from kernel space..

> Structure for event records :
> 
> struct ring_buffer_event {
>         u32 type:2, len:3, time_delta:27;
>         u32 array[];
> };
> 
> The only good thing about reserving 2 bits for event IDs in there is to
> put the most frequent events in those IDs

Not so, these types are buffer package types, not actual event types, I
think thats a useful distinction.

> , which is clearly not the
> case:
> RB_TYPE_PADDING: utterly useless. Can be expressed by a sub-buffer
> header telling the length of data written into the subbuffer (what you
> guys call a "page", but what I still think might be worthy to be of
> variable size, especially with a light locking infrastructure and
> considering we might want to export this data to userspace).

But then you'd need a sub-buffer header, which in itself takes space,
this padding solution seems like a fine middle-ground, it only takes
space when you need it and it free otherwise.

The sub-buffer headers would always take space.

> RB_TYPE_TIME_EXTENT : I'd reserve a separate bit for this one.
> 
> Also, if size _really_ matters,

You and Martin have been telling it does ;-)

>  we should just export the event ID and
> look up the event payload size through a separate table. If the payload
> consists of a binary blob, then the data structure should start by a
> payload size and then have a the actual binary blob.
> 
> struct ring_buffer_event {
>         u32 time_ext:1, evid:4, time_lsb:27;
>         union {
>           u32 array[];
>           struct {
>             u32 ext_id;
>             u32 array[];
>           };
>           struct {
>             u32 ext_time;
>             u32 array[];
>           };
>           struct {
>             u32 ext_time;
>             u32 ext_id;
>             u32 array[];
>           };
> };
> 
> Therefore we can encode up to 15 event IDs into this compact
> representation (we reserve ID 0xF for extended id). If we assign those
> IDs per subbuffer, it leaves plenty of room before we have to write a
> supplementary field for more IDs.
> 
> OTOH, if we really want to have an event size in there (which is more
> solid), we could have :
> 
> struct ring_buffer_event {
>         u32 time_ext:1, time_lsb:31;
>         u16 evid;
>         u16 evsize;
>         union {
>           u32 array[];
>           struct {
>             u32 ext_time;
>             u32 array[];
>           };
> };
> 
> That's a bit bigger, but keeps the event size in the data stream.

I'm really not seeing what any of these proposals have on top of what
Steve currently has. We have the ability to encode up to 28 bytes of
payload in a 4 bytes header, which should suffice for most entries,
right?

> Also, nobody has explained successfully why we have to encode a time
> _delta_ (current tsc - prev tsc) rather than just putting the LSBs. So
> either I fail to see the light here (I doubt it), or I am not clear
> enough when I say that we can just put the raw LSBs and compute the
> delta afterward when reading the buffers, manage to keep the same
> overflow detection power, and also keep the absolute value of the tsc
> lsb, which makes it much easier to cross-check than "deltas".

Still not quite understanding where you get the MSBs from, how do you
tell if two LSBs are from the same period?

> Now for the buffer pages implementation :
> 
> 
> +struct buffer_page {
> +       union {
> +               struct {
> +                       unsigned long flags;    /* mandatory */
> +                       atomic_t _count;        /* mandatory */
> +                       u64     time_stamp;     /* page time stamp */
> 
> Why should we ever associate a time stamp with a page ??

Discarting the whole sub-buffer idea, it could be used to validate
whichever time-stamp logic.

Personally I don't particulary like the sub-buffer concept, and I don't
think we need it.

> I see that we could save the timestamp at which a subbuffer switch
> happens (which in this patchset semantics happens to be a page), but why
> would we every want to save that in the page frame ? Especially if we
> simply write the LSBs instead of a time delta... Also, I would write
> this timestamp in a subbuffer _header_ which is exported to userspace,

Why have sub-buffers at all?

> but I clealry don't see why we keep it here. In addition, it's
> completely wrong in a layered approach : if we want to switch from pages
> to video memory or to a statically allocated buffer at boot time, such
> "page-related" timestamp hacks won't do.

I don't think you _ever_ want to insert actual video memory in the
trace, what you _might_ possibly want to do, is insert a copy of a
frame, but that you can do with paged buffers like we have, just add an
entry with 3840*1200*4 bytes (one screen in my case), and use sub-writes
to copy everything to page-alinged chunks.

There is nothing techinically prohibiting this in Steve's scheme, except
for Linus telling you you're an idiot for doing it.

The NOP packets you dislike allow us to align regular entries with page
boundaries, which in turn allows this 0-copy stuff. If you use the
sub-write iterator stuff we taked about a while ago, you can leave out
the NOPs.

Having both makes sense (if you want the large packets stuff), use the
regular page aligned 0-copy stuff for regular small packets, and use the
sub-write iterator stuff for your huge packets.

> > As for the page-spanning entries, I think we can do that with Steve's
> > system just fine, its just that Linus said its a dumb idea and Steve
> > dropped it, but there is nothing fundamental stopping us from recording
> > a length that is > PAGE_SIZE and copying data into the pages one at a
> > time.
> > 
> > Nor do I see it impossible to implement splice on top of Steve's
> > ring-buffer..
> > 
> > So again, why?
> > 
> 
> I'd like to separate the layer which deals with data read/write from the
> layer which deals with synchronization of data write/read to/from the
> buffers so we can eventually switch to a locking mechanism which
> provides a sane performance level, and given Steven's linked list
> implementation, it will just add unneeded locking requirements.

How does it differ from your linked list implementation? Reaslistically,
we want but a single locking scheme for the trace buffer stuff. So
factoring it out doesn't really make sense.

> Compared to this, I deal with buffer coherency with two 32 bits counters
> in LTTng : a write offset and a consumer offset. (plus a per-subbuffer
> commit count). 

I think that can work just fine on top of Steve's stuff too, it needs a
bit of trimming etc.. but afaict there isn't anything stopping us from
implementing his reserve function as light as you want:

int reserve(buffer, size, flags)
{
 preempt_disable()
 cpu = smp_processor_id();
 cpu_buf = buffer->buffers[cpu];

 if (cpu_buf->stop) {
   ret = -EBUSY;
   goto out;
 }

again:
 pos = cpu_buf->write_pos;
 if (flags & CONTIG) {
   new_pos = pos + size;
 } else {
   if (size > PAGE_SIZE) {
     ret = -ENOSPACE;
     goto out;
   }
   if ((pos + size) & PAGE_MASK != pos & PAGE_MASK) {
     insert_nops();
   }
   new_pos = pos + size;
 }
 if (cmpxchg(&cpu_buf->write_pos, pos, new_pos) != pos)
   goto again;
 return pos;

out:
 preempt_enable();
 return ret;
}

If you put the offset&PAGE_MASK in each page-frame you can use that to
easily detect when you need to flip to the next page.

Which I imagine is similar to what you have... although I must admit to
not being sure how to deal with over-write here, I guess your buffer
must be large enough to ensure no nesting depth allows you to
wrap-around while having an even un-commited.

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