Re: [RFC PATCH 1/3] Unified trace buffer
From: Mathieu Desnoyers
Date: Wed Sep 24 2008 - 12:14:21 EST
* Martin Bligh (mbligh@xxxxxxxxxx) wrote:
> Thanks for creating this so quickly ;-)
>
> >> We can record either the fast way of reserving a part of the buffer:
> >>
> >> event = ring_buffer_lock_reserve(buffer, event_id, length, &flags);
> >> event->data = record_this_data;
> >> ring_buffer_unlock_commit(buffer, event, flags);
> >
> > This can, in generic, not work. Due to the simple fact that we might
> > straddle a page boundary. Therefore I think its best to limit our self
> > to the write interface below, so that it can handle that.
>
> I'm not sure why this is any harder to deal with in write, than it is
> in reserve? We should be able to make reserve handle this just
> as well?
>
> If you use write rather than reserve, you have to copy all the data
> twice for every event.
>
I think we all agree that a supplementary copy is no wanted, but I think
this question is orthogonal to having a write wrapper. The way we can do
both is by using reserve/commit to deal with space reservation, and a
write() to perform the actual data write into the buffers once space has
been reserved.
Reserve/commit would allocate a variable-sized "slot" into the buffer.
We reserve X amount of bytes, and it returns the offset from the
buffer start where the allocated slot is. This reserve/commit mechanism
deals with synchronization (cli/spinlock or cmpxchg_local scheme...).
We can then use this offset to see in which page(s) we have to write.
This offset + len can in fact cross multiple page boundaries.
Doing this elegantly could involve a page array that would represent the
buffer data :
struct page **buffer;
And be given as parameter to the read() and write() methods, which would
deal with page-crossing.
e.g.
/*
* Perform an aligned write of the input data into the buffer.
*
* buffer : page pointer array
* woffset : offset in the page pointer array where write starts from
* data : input data
* len : length of data to copy
*
* Note : if a NULL buffer is passed, no copy is performed, but the
* alignment and offset calculation is done. Useful to calculate the
* size to reserve.
*
* return : length written
*/
size_t write(struct page **buffer, size_t woffset, void *data, size_t len);
Therefore, we could have code which writes in the buffers, without extra
copy, and without using vmap, in multiple writes for a single event,
which would deal with data alignment, e.g. :
size_t woffset, evsize = 0;
evsize += write(NULL, evsize, &var1, sizeof(var1));
evsize += write(NULL, evsize, &var2, sizeof(var2));
evsize += write(NULL, evsize, &var3, sizeof(var3));
woffset = reserve(..., evsize);
woffset += write(buffer, woffset, &var1, sizeof(var1));
woffset += write(buffer, woffset, &var2, sizeof(var2));
woffset += write(buffer, woffset, &var3, sizeof(var3));
commit(..., evsize);
Does that make sense ?
Mathieu
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Mathieu Desnoyers
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