On 20.04.2012 00:03, Luigi Rizzo wrote:
On Thu, Apr 19, 2012 at 11:20:00PM +0200, Andre Oppermann wrote:
On 19.04.2012 22:46, Luigi Rizzo wrote:
The allocation happens while the code has already an exclusive
lock on so->snd_buf so a pool of fresh buffers could be attached
there.
Ah, there it is not necessary to hold the snd_buf lock while
doing the allocate+copyin. With soreceive_stream() (which is
it is not held in the tx path either -- but there is a short section
before m_uiotombuf() which does
...
SOCKBUF_LOCK(&so->so_snd);
// check for pending errors, sbspace, so_state
SOCKBUF_UNLOCK(&so->so_snd);
...
(some of this is slightly dubious, but that's another story)
Indeed the lock isn't held across the m_uiotombuf(). You're talking
about filling an sockbuf mbuf cache while holding the lock?
But the other consideration is that one could defer the mbuf allocation
to a later time when the packet is actually built (or anyways
right before the thread returns).
What i envision (and this would fit nicely with netmap) is the following:
- have a (possibly readonly) template for the headers (MAC+IP+UDP)
attached to the socket, built on demand, and cached and managed
with similar invalidation rules as used by fastforward;
That would require to cross-pointer the rtentry and whatnot again.
i was planning to keep a copy, not a reference. If the copy becomes
temporarily stale, no big deal, as long as you can detect it reasonably
quiclky -- routes are not guaranteed to be correct, anyways.
Be wary of disappearing interface pointers...
- possibly extend the pru_send interface so one can pass down the uio
instead of the mbuf;
- make an opportunistic buffer allocation in some place downstream,
where the code already has an x-lock on some resource (could be
the snd_buf, the interface, ...) so the allocation comes for free.
ETOOCOMPLEXOVERTIME.
maybe. But i want to investigate this.
I fail see what passing down the uio would gain you. The snd_buf lock
isn't obtained again after the copyin. Not that I want to prevent you
from investigating other ways. ;)
--
Andre
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