On Tuesday 10 October 2006 10:12, Eric Dumazet wrote:
> On Friday 06 October 2006 06:51, Vadim Lobanov wrote:
> > - nfds = max_t(int, 8 * L1_CACHE_BYTES, roundup_pow_of_two(nr + 1));
> > - if (nfds > NR_OPEN)
> > - nfds = NR_OPEN;
> > -
> > - new_openset = alloc_fdset(nfds);
> > - new_execset = alloc_fdset(nfds);
> > - if (!new_openset || !new_execset)
> > - goto out;
> > - fdt->open_fds = new_openset;
> > - fdt->close_on_exec = new_execset;
> > - fdt->max_fdset = nfds;
> > -
> > nfds = NR_OPEN_DEFAULT;
> > /*
> > * Expand to the max in easy steps, and keep expanding it until
> > @@ -271,15 +254,21 @@ static struct fdtable *alloc_fdtable(int
> > nfds = NR_OPEN;
> > }
> > } while (nfds <= nr);
>
> If I understand well, we may allocate very small fdset, while previous
> minimum size was L1_CACHE_BYTES bytes. (512 bits for a 64 bytes cache line)
>
> If you check commit 0c9e63fd38a2fb2181668a0cdd622a3c23cfd567,
> (http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=com
>mit;h=0c9e63fd38a2fb2181668a0cdd622a3c23cfd567 ) you'll find this comment of
> mine :
>
> 3) Reduce size of allocated fdset. Currently two full pages are
> allocated, that is 32768 bits on x86 for example, and way too much. The
> minimum is now L1_CACHE_BYTES.
>
> This minimum is mandatory to be sure two tasks wont share the same cache
> line to store their fdset (and possibly do lot of cache line ping pongs)
Good point. This should be easy to fix -- we can simply bump up the allocation
'nr' when calling alloc_fdmem() for the fdset. Since we won't cross a page
size, the rest of the code will still stay the same.
I'll code this up once I track down why other folks are experiencing oopses.
> Eric
-- Vadim Lobanov
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