Messages in this thread | | | Date | Wed, 24 Sep 2008 12:13:47 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 1/3] Unified trace buffer |
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* Martin Bligh (mbligh@google.com) 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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