[SPARC64]: Fix and re-enable dynamic TSB sizing.

This is good for up to %50 performance improvement of some test cases.
The problem has been the race conditions, and hopefully I've plugged
them all up here.

1) There was a serious race in switch_mm() wrt. lazy TLB
   switching to and from kernel threads.

   We could erroneously skip a tsb_context_switch() and thus
   use a stale TSB across a TSB grow event.

   There is a big comment now in that function describing
   exactly how it can happen.

2) All code paths that do something with the TSB need to be
   guarded with the mm->context.lock spinlock.  This makes
   page table flushing paths properly synchronize with both
   TSB growing and TLB context changes.

3) TSB growing events are moved to the end of successful fault
   processing.  Previously it was in update_mmu_cache() but
   that is deadlock prone.  At the end of do_sparc64_fault()
   we hold no spinlocks that could deadlock the TSB grow
   sequence.  We also have dropped the address space semaphore.

While we're here, add prefetching to the copy_tsb() routine
and put it in assembler into the tsb.S file.  This piece of
code is quite time critical.

There are some small negative side effects to this code which
can be improved upon.  In particular we grab the mm->context.lock
even for the tsb insert done by update_mmu_cache() now and that's
a bit excessive.  We can get rid of that locking, and the same
lock taking in flush_tsb_user(), by disabling PSTATE_IE around
the whole operation including the capturing of the tsb pointer
and tsb_nentries value.  That would work because anyone growing
the TSB won't free up the old TSB until all cpus respond to the
TSB change cross call.

I'm not quite so confident in that optimization to put it in
right now, but eventually we might be able to and the description
is here for reference.

This code seems very solid now.  It passes several parallel GCC
bootstrap builds, and our favorite "nut cruncher" stress test which is
a full "make -j8192" build of a "make allmodconfig" kernel.  That puts
about 256 processes on each cpu's run queue, makes lots of process cpu
migrations occur, causes lots of page table and TLB flushing activity,
incurs many context version number changes, and it swaps the machine
real far out to disk even though there is 16GB of ram on this test
system. :-)

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 38 insertions, 12 deletions

diff --git a/include/asm-sparc64/mmu_context.h b/include/asm-sparc64/mmu_context.h
index ca36ea96f64b..e7974321d052 100644
--- a/include/asm-sparc64/mmu_context.h
+++ b/include/asm-sparc64/mmu_context.h
@@ -42,7 +42,7 @@ static inline void tsb_context_switch(struct mm_struct *mm)
                              __pa(&mm->context.tsb_descr));
 }
 
-extern void tsb_grow(struct mm_struct *mm, unsigned long mm_rss, gfp_t gfp_flags);
+extern void tsb_grow(struct mm_struct *mm, unsigned long mm_rss);
 #ifdef CONFIG_SMP
 extern void smp_tsb_sync(struct mm_struct *mm);
 #else
@@ -74,18 +74,43 @@ static inline void switch_mm(struct mm_struct *old_mm, struct mm_struct *mm, str
         ctx_valid = CTX_VALID(mm->context);
         if (!ctx_valid)
                 get_new_mmu_context(mm);
-        spin_unlock_irqrestore(&mm->context.lock, flags);
 
-        if (!ctx_valid || (old_mm != mm)) {
-                load_secondary_context(mm);
-                tsb_context_switch(mm);
-        }
+        /* We have to be extremely careful here or else we will miss
+         * a TSB grow if we switch back and forth between a kernel
+         * thread and an address space which has it's TSB size increased
+         * on another processor.
+         *
+         * It is possible to play some games in order to optimize the
+         * switch, but the safest thing to do is to unconditionally
+         * perform the secondary context load and the TSB context switch.
+         *
+         * For reference the bad case is, for address space "A":
+         *
+         *              CPU 0                   CPU 1
+         *      run address space A
+         *      set cpu0's bits in cpu_vm_mask
+         *      switch to kernel thread, borrow
+         *      address space A via entry_lazy_tlb
+         *                                      run address space A
+         *                                      set cpu1's bit in cpu_vm_mask
+         *                                      flush_tlb_pending()
+         *                                      reset cpu_vm_mask to just cpu1
+         *                                      TSB grow
+         *      run address space A
+         *      context was valid, so skip
+         *      TSB context switch
+         *
+         * At that point cpu0 continues to use a stale TSB, the one from
+         * before the TSB grow performed on cpu1.  cpu1 did not cross-call
+         * cpu0 to update it's TSB because at that point the cpu_vm_mask
+         * only had cpu1 set in it.
+         */
+        load_secondary_context(mm);
+        tsb_context_switch(mm);
 
-        /* Even if (mm == old_mm) we _must_ check
-         * the cpu_vm_mask.  If we do not we could
-         * corrupt the TLB state because of how
-         * smp_flush_tlb_{page,range,mm} on sparc64
-         * and lazy tlb switches work. -DaveM
+        /* Any time a processor runs a context on an address space
+         * for the first time, we must flush that context out of the
+         * local TLB.
          */
         cpu = smp_processor_id();
         if (!ctx_valid || !cpu_isset(cpu, mm->cpu_vm_mask)) {
@@ -93,6 +118,7 @@ static inline void switch_mm(struct mm_struct *old_mm, struct mm_struct *mm, str
                 __flush_tlb_mm(CTX_HWBITS(mm->context),
                                SECONDARY_CONTEXT);
         }
+        spin_unlock_irqrestore(&mm->context.lock, flags);
 }
 
 #define deactivate_mm(tsk,mm)   do { } while (0)
@@ -109,11 +135,11 @@ static inline void activate_mm(struct mm_struct *active_mm, struct mm_struct *mm
         cpu = smp_processor_id();
         if (!cpu_isset(cpu, mm->cpu_vm_mask))
                 cpu_set(cpu, mm->cpu_vm_mask);
-        spin_unlock_irqrestore(&mm->context.lock, flags);
 
         load_secondary_context(mm);
         __flush_tlb_mm(CTX_HWBITS(mm->context), SECONDARY_CONTEXT);
         tsb_context_switch(mm);
+        spin_unlock_irqrestore(&mm->context.lock, flags);
 }
 
 #endif /* !(__ASSEMBLY__) */