Re: sched_lock && thread_lock()

On Tue, 22 May 2007, Bruce Evans wrote:

Bugs found while grepping for sched_locking of VMCNT fields:
- v_trap is supposed to be updated by PCPU_LAZY_INC(), but subr_trap.c
updates it by VMCNT_ADD(). The direct access used to be perfectly unlocked
and non-atomic, since it occurs immediately after releasing sched_lock
and just used "++". Now it is atomic and a micro-pessimization.
- v_trap's name is now hidden by the macro so grepping for it doesn't work.

I'm still searching for a 1% performance loss in makeworld under UP
that occurred just after the VMCNT changes. I've only found this
harmless (?) bug so far:

% Index: vnode_pager.c
% ===================================================================
% RCS file: /home/ncvs/src/sys/vm/vnode_pager.c,v
% retrieving revision 1.232
% retrieving revision 1.233
% diff -u -2 -r1.232 -r1.233
% --- vnode_pager.c 14 Oct 2006 23:21:48 -0000 1.232
% +++ vnode_pager.c 18 May 2007 07:10:50 -0000 1.233
% @@ -910,6 +910,6 @@
% atomic_add_int(&runningbufspace, bp->b_runningbufspace);
% % - cnt.v_vnodein++;
% - cnt.v_vnodepgsin += count;
^^^^^
% + VMCNT_ADD(vnodein, 1);
% + VMCNT_ADD(vnodepgsin, 1);
^
% % /* do the input */

Related API problem: v_vnodein and v_vnodepgsin are not used in the kernel
except to increment them and to export them using sysctl, so they should
be incremented using PCPY_LAZY_INC(). However, PCPU_LAZY_INC() can only
do increments of 1.

The increments of +1 also complicate searching for the regression. After
removing `volatile', the VMCNT change should have no effect on the binary,
but it has an effect since increments of 1 are replaced by "movl $1,%reg;
addl %reg,mem" in i386 code, since the i386 atomic asms don't use incl
and the "ir" constraint isn't working as intended (this is with gcc-3.4.6).
The "ir" constraint should result in the "i" variant being chosen and
generate "addl $1,mem".

Bruce
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