Re: [PATCH 3/4] flowcache: make struct flow_cache_percpu::hash_rnd_recalc bool

[Alexey Dobriyan]

On Wed, Mar 22, 2017 at 5:09 AM, David Miller  wrote:
> From: Alexey Dobriyan 
> Date: Mon, 20 Mar 2017 01:27:43 +0300
>
>> ->hash_rnd_recalc is only used in boolean context.
>>
>> Space savings on x86_64 come from the fact that "MOV rm8, imm8" is
>> shorter than "MOV rm32, imm32" by at least 3 bytes.
>>
>>   add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-10 (-10)
>>   function old new   delta
>>   flow_cache_new_hashrnd   166 163  -3
>>   flow_cache_cpu_up_prep   171 168  -3
>>   flow_cache_lookup   11481144  -4
>>   Total: Before=170822872, After=170822862, chg -0.00%
>>
>> Signed-off-by: Alexey Dobriyan 
>
> I agree with Eric Dumazet that we might have atomicity issues in the
> future because of this change.
>
> Why don't you drop this and resubmit just the other 3 patches which
> seem to be much less controversial?

Sure, bool patch was last minute change.

Re: [PATCH 3/4] flowcache: make struct flow_cache_percpu::hash_rnd_recalc bool

[David Miller]

From: Alexey Dobriyan 
Date: Mon, 20 Mar 2017 01:27:43 +0300

> ->hash_rnd_recalc is only used in boolean context.
> 
> Space savings on x86_64 come from the fact that "MOV rm8, imm8" is
> shorter than "MOV rm32, imm32" by at least 3 bytes.
> 
>   add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-10 (-10)
>   function old new   delta
>   flow_cache_new_hashrnd   166 163  -3
>   flow_cache_cpu_up_prep   171 168  -3
>   flow_cache_lookup   11481144  -4
>   Total: Before=170822872, After=170822862, chg -0.00%
> 
> Signed-off-by: Alexey Dobriyan 

I agree with Eric Dumazet that we might have atomicity issues in the
future because of this change.

Why don't you drop this and resubmit just the other 3 patches which
seem to be much less controversial?

Thanks.

Re: [PATCH 3/4] flowcache: make struct flow_cache_percpu::hash_rnd_recalc bool

[Eric Dumazet]

On Mon, 2017-03-20 at 01:27 +0300, Alexey Dobriyan wrote:
> ->hash_rnd_recalc is only used in boolean context.
> 
> Space savings on x86_64 come from the fact that "MOV rm8, imm8" is
> shorter than "MOV rm32, imm32" by at least 3 bytes.
> 
>   add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-10 (-10)
>   function old new   delta
>   flow_cache_new_hashrnd   166 163  -3
>   flow_cache_cpu_up_prep   171 168  -3
>   flow_cache_lookup   11481144  -4
>   Total: Before=170822872, After=170822862, chg -0.00%
> 
> Signed-off-by: Alexey Dobriyan 
> ---
> 
>  include/net/flowcache.h |2 +-
>  net/core/flow.c |6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> --- a/include/net/flowcache.h
> +++ b/include/net/flowcache.h
> @@ -10,7 +10,7 @@ struct flow_cache_percpu {
>   struct hlist_head   *hash_table;
>   int hash_count;
>   u32 hash_rnd;
> - int hash_rnd_recalc;
> + boolhash_rnd_recalc;

hash_rnd_recalc can be written from flow_cache_new_hashrnd() without any
locking.

Some arches do not have the ability to store an u8 atomically.

So your patch adds a possibility that in the future, we might have a
bug, if another field is added there.

Basically the whole integer here should be reserved, or converted to a
real flag (that can be manipulated with clear_bit() and set_bit()),
but this would consume 8 bytes instead of 4 ;)

[PATCH 3/4] flowcache: make struct flow_cache_percpu::hash_rnd_recalc bool

[Alexey Dobriyan]

->hash_rnd_recalc is only used in boolean context.

Space savings on x86_64 come from the fact that "MOV rm8, imm8" is
shorter than "MOV rm32, imm32" by at least 3 bytes.

add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-10 (-10)
function old new   delta
flow_cache_new_hashrnd   166 163  -3
flow_cache_cpu_up_prep   171 168  -3
flow_cache_lookup   11481144  -4
Total: Before=170822872, After=170822862, chg -0.00%

Signed-off-by: Alexey Dobriyan 
---

 include/net/flowcache.h |2 +-
 net/core/flow.c |6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

--- a/include/net/flowcache.h
+++ b/include/net/flowcache.h
@@ -10,7 +10,7 @@ struct flow_cache_percpu {
struct hlist_head   *hash_table;
int hash_count;
u32 hash_rnd;
-   int hash_rnd_recalc;
+   boolhash_rnd_recalc;
struct tasklet_struct   flush_tasklet;
 };
 
--- a/net/core/flow.c
+++ b/net/core/flow.c
@@ -56,7 +56,7 @@ static void flow_cache_new_hashrnd(unsigned long arg)
int i;
 
for_each_possible_cpu(i)
-   per_cpu_ptr(fc->percpu, i)->hash_rnd_recalc = 1;
+   per_cpu_ptr(fc->percpu, i)->hash_rnd_recalc = true;
 
fc->rnd_timer.expires = jiffies + FLOW_HASH_RND_PERIOD;
add_timer(>rnd_timer);
@@ -155,7 +155,7 @@ static void flow_new_hash_rnd(struct flow_cache *fc,
  struct flow_cache_percpu *fcp)
 {
get_random_bytes(>hash_rnd, sizeof(u32));
-   fcp->hash_rnd_recalc = 0;
+   fcp->hash_rnd_recalc = false;
__flow_cache_shrink(fc, fcp, 0);
 }
 
@@ -412,7 +412,7 @@ static int flow_cache_cpu_prepare(struct flow_cache *fc, 
int cpu)
pr_err("NET: failed to allocate flow cache sz %u\n", 
sz);
return -ENOMEM;
}
-   fcp->hash_rnd_recalc = 1;
+   fcp->hash_rnd_recalc = true;
fcp->hash_count = 0;
tasklet_init(>flush_tasklet, flow_cache_flush_tasklet, 0);
}