В Вт, 08/03/2022 в 15:40 +0000, Matthew Wilcox пишет:
> On Tue, Mar 08, 2022 at 06:16:35PM +0300, Ananda Badmaev wrote:
> > > Hm. So 368 bytes on 64-bit, but 372 bytes on 32-bit? Is that
> > > intentional? Why 11?
> > >
> > Yes, 'slot_size' and 'slots_per_block' values are chosen so that in
> > general the range from 0 to PAGE_SIZE is split more or less evenly
> > and
> > the size of the block is as close as possible to the power of 2.
> > Also
> > 'slot_size' values are aligned to the size of long.
>
> Is it intrinsic to the design that the blocks are aligned to
> sizeof(long)?
slot_size values must be aligned so that the slot addresses in blocks
are also algined to avoid potential problems with unalinged access. I
guess 4/8 byte alignment is enough for 32/64 bit systems respectively.
> > >
> > > You have some very strange indentation (throughout).
> > >
> > I was trying to limit the length of lines.
>
> But you didn't achieve that. The _start_ of the line containing
> block_type and gfp was _more indented_ than the end of the previous
> line. Do you have unusual tab settings?
>
Maybe it's because I have tab size 4 in my editor.
> > > thinking
> > > about it". What if you have two threads that execute this path
> > > concurrently? Looks to me like you leak the memory allocated by
> > > the
> > > first thread.
> > >
> > Probably I should pass GFP_ATOMIC flag to alloc_block() and execute
> > this entire section of code under single spinlock.
>
> I think that's a bad approach. Better would be:
>
> spin_lock();
> if (free_slot_block)
> goto found;
> spin_unlock();
> block = alloc_block();
> spin_lock();
> if (free_slot_block) {
> free_block(block);
> block = free_slot_block;
> } else {
> free_slot_block = block;
> }
> found:
> ...
> spin_unlock();
Yes, seems really better to do like this. Solution with GFP_ATOMIC
probably will improve the worst time of ztree_alloc(), which was not
big issue in the tests, but most likely general efficiency will be
degraded.
