On 09/11/2015 04:37 PM, Linus Torvalds wrote:
On Fri, Sep 11, 2015 at 1:02 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
How about we instead:
(a) revert that commit d353d7587 as broken (because it clearly is)
(b) add a big honking comment about the fact that we hold 'list_lock'
in writeback_sb_inodes()
(c) move the plugging up to wb_writeback() and writeback_inodes_wb()
_outside_ the spinlock.
Ok, I've done (a) and (b) in my tree. And attached is the totally
untested patch for (c). It looks ObviouslyCorrect(tm), but since this
is a performance issue, I'm not going to commit it without some more
ACK's from people.
I obviously think this is a *much* better approach than dropping and
retaking the lock, but there might be something silly I'm missing.
For example, maybe we want to unplug and replug around the
"inode_sleep_on_writeback()" in wb_writeback()? So while the revert
was a no-brainer, this one I really want people to think about.
So we talked about this when we were trying to figure out a solution.
The problem with this approach is now we have a plug that covers
multiple super blocks (__writeback_inodes_wb loops through the sb's
starts writeback), which is likely to give us crappier performance than
no plug at all. Thanks,
Josef
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