Re: [uml-devel] [PATCH 2/2] UML - Speed up exec

[Blaisorblade <blaisorblade@yahoo.it>]

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On venerdì 30 marzo 2007, Jeff Dike wrote:
> flush_thread doesn't need to do a full page table walk in order to
> clear the address space.  It knows what the end result needs to be, so
> it can call unmap directly.
>
> This results in a 10-20% speedup in an exec from bash.

Oh, yeah!
When porting part of Ingo's work, I realized that a similar thing can be done 
for fork().

If the whole address space is unmapped in init_new_context_skas(), the first 
fix_range_common() call won't need to call unmap at all. He did this with 
remap_file_pages(), where init_new_context_skas() must "unmap" everything 
anyway.

This is giving some speedup in lmbench (5% better in fork proc, 2% better in 
exec proc), but the results are still controversial, there is one benchmark 
with a 2% slowdown (called 'mmap latency').

In a loop, it maps, touches a byte per page and unmaps a region with growing 
size (up to 32MB).

However, since results aren't yet stable for some other benchmark (context 
switching benchmark is crazy), I'm still studying on this.

> Signed-off-by: Jeff Dike <jdike@linux.intel.com>
> --
>  arch/um/kernel/skas/exec.c |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> Index: linux-2.6.21-mm/arch/um/kernel/skas/exec.c
> ===================================================================
> --- linux-2.6.21-mm.orig/arch/um/kernel/skas/exec.c	2007-03-30
> 10:28:24.000000000 -0400 +++
> linux-2.6.21-mm/arch/um/kernel/skas/exec.c	2007-03-30 10:30:15.000000000
> -0400 @@ -17,7 +17,17 @@
>
>  void flush_thread_skas(void)
>  {
> -	force_flush_all();
> +	void *data = NULL;
> +	unsigned long end = proc_mm ? task_size : CONFIG_STUB_START;
> +	int ret;
> +
> +	ret = unmap(&current->mm->context.skas.id, 0, end, 1, &data);
> +	if(ret){
> +		printk("flush_thread_skas - clearing address space failed, "
> +		       "err = %d\n", ret);
> +		force_sig(SIGKILL, current);
> +	}
> +
>  	switch_mm_skas(&current->mm->context.skas.id);
>  }


-- 
Inform me of my mistakes, so I can add them to my list!
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade

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Index: linux-2.6.git/arch/um/include/skas/mmu-skas.h
===================================================================
--- linux-2.6.git.orig/arch/um/include/skas/mmu-skas.h
+++ linux-2.6.git/arch/um/include/skas/mmu-skas.h
@@ -16,6 +16,7 @@ struct mmu_context_skas {
 	unsigned long last_pmd;
 #endif
 	uml_ldt_t ldt;
+	int first_flush;
 };
 
 extern void switch_mm_skas(struct mm_id * mm_idp);
Index: linux-2.6.git/arch/um/kernel/skas/mmu.c
===================================================================
--- linux-2.6.git.orig/arch/um/kernel/skas/mmu.c
+++ linux-2.6.git/arch/um/kernel/skas/mmu.c
@@ -77,6 +77,7 @@ int init_new_context_skas(struct task_st
 	struct mmu_context_skas *to_mm = &mm->context.skas;
 	unsigned long stack = 0;
 	int ret = -ENOMEM;
+	void *unused = NULL;
 
 	if(skas_needs_stub){
 		stack = get_zeroed_page(GFP_KERNEL);
@@ -121,6 +122,14 @@ int init_new_context_skas(struct task_st
 		else to_mm->id.u.pid = start_userspace(stack);
 	}
 
+	mm->context.skas.first_flush = 1;
+	ret = unmap(&mm->context.skas.id, 0, TASK_SIZE, 1, &unused);
+	if (ret < 0) {
+		printk("init_new_context_skas - unmap failed, "
+		       "errno = %d; continuing\n", ret);
+		mm->context.skas.first_flush = 0;
+	}
+
 	ret = init_new_ldt(to_mm, from_mm);
 	if(ret < 0){
 		printk("init_new_context_skas - init_ldt"
Index: linux-2.6.git/arch/um/kernel/tlb.c
===================================================================
--- linux-2.6.git.orig/arch/um/kernel/tlb.c
+++ linux-2.6.git/arch/um/kernel/tlb.c
@@ -139,10 +139,17 @@ void fix_range_common(struct mm_struct *
 	void *flush = NULL;
 	int op_index = -1, last_op = ARRAY_SIZE(ops) - 1;
 	int ret = 0;
+	int first_flush;
 
 	if(mm == NULL)
 		return;
 
+	/* Nothing is mapped in this address space, so no call to add_munmap()
+	 * must be done */
+	first_flush = mm->context.skas.first_flush;
+
+	mm->context.skas.first_flush = 0;
+
 	ops[0].type = NONE;
 	for(addr = start_addr; addr < end_addr && !ret;){
 		npgd = pgd_offset(mm, addr);
@@ -151,9 +158,10 @@ void fix_range_common(struct mm_struct *
 			if(end > end_addr)
 				end = end_addr;
 			if(force || pgd_newpage(*npgd)){
-				ret = add_munmap(addr, end - addr, ops,
-						 &op_index, last_op, mmu,
-						 &flush, do_ops);
+				if (!first_flush)
+					ret = add_munmap(addr, end - addr, ops,
+							 &op_index, last_op, mmu,
+							 &flush, do_ops);
 				pgd_mkuptodate(*npgd);
 			}
 			addr = end;
@@ -166,9 +174,10 @@ void fix_range_common(struct mm_struct *
 			if(end > end_addr)
 				end = end_addr;
 			if(force || pud_newpage(*npud)){
-				ret = add_munmap(addr, end - addr, ops,
-						 &op_index, last_op, mmu,
-						 &flush, do_ops);
+				if (!first_flush)
+					ret = add_munmap(addr, end - addr, ops,
+							 &op_index, last_op, mmu,
+							 &flush, do_ops);
 				pud_mkuptodate(*npud);
 			}
 			addr = end;
@@ -181,9 +190,10 @@ void fix_range_common(struct mm_struct *
 			if(end > end_addr)
 				end = end_addr;
 			if(force || pmd_newpage(*npmd)){
-				ret = add_munmap(addr, end - addr, ops,
-						 &op_index, last_op, mmu,
-						 &flush, do_ops);
+				if (!first_flush)
+					ret = add_munmap(addr, end - addr, ops,
+							 &op_index, last_op, mmu,
+							 &flush, do_ops);
 				pmd_mkuptodate(*npmd);
 			}
 			addr = end;
@@ -207,7 +217,8 @@ void fix_range_common(struct mm_struct *
 					       PAGE_SIZE, r, w, x, ops,
 					       &op_index, last_op, mmu,
 					       &flush, do_ops);
-			else ret = add_munmap(addr, PAGE_SIZE, ops,
+			else if (!first_flush)
+				ret = add_munmap(addr, PAGE_SIZE, ops,
 					      &op_index, last_op, mmu,
 					      &flush, do_ops);
 		}
@@ -219,7 +230,7 @@ void fix_range_common(struct mm_struct *
 		*npte = pte_mkuptodate(*npte);
 		addr += PAGE_SIZE;
 	}
-	if(!ret)
+	if (!ret && op_index > -1)
 		ret = (*do_ops)(mmu, ops, op_index, 1, &flush);
 
 /* This is not an else because ret is modified above */

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