linux-kernel - Re: [PATCH ipmi/kcs_bmc v1] ipmi: kcs_bmc: optimize the data buffers allocation

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Message-ID: <dd4e07c4-7a99-3449-fd2d-58c2b829f7b7@linux.intel.com>
Date:   Sat, 7 Apr 2018 10:37:22 +0800
From:   "Wang, Haiyue" <haiyue.wang@...ux.intel.com>
To:     minyard@....org, openipmi-developer@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH ipmi/kcs_bmc v1] ipmi: kcs_bmc: optimize the data buffers
 allocation



On 2018-04-07 05:47, Corey Minyard wrote:
> On 03/15/2018 07:20 AM, Haiyue Wang wrote:
>> Allocate a continuous memory block for the three KCS data buffers with
>> related index assignment.
>
> I'm finally getting to this.
>
> Is there a reason you want to do this?  In general, it's better to not 
> try to
> outsmart your base system.  Depending on the memory allocator, in this
> case, you might actually use more memory.  You probably won't use any
> less.
>
I got this idea from another code review, but that patch allocates 30 more
the same size memory block, reducing the devm_kmalloc call will be better.
For KCS only have 3, may be the key point is memory waste.

> In the original case, you allocate three 1000 byte buffers, resulting 
> in 3
> 1024 byte slab allocated.
>
> In the changed case, you will allocate a 3000 byte buffer, resulting in
> a single 4096 byte slab allocation, wasting 1024 more bytes of memory.
>
As the kcs has memory copy between in/out/kbuffer, put them in the same
page will be better ? Such as the same TLB ? (Well, I just got this from 
book,
no real experience of memory accessing performance. And also, I was told
that using space to save the time. :-)).

Just my stupid thinking. I'm OK to drop this patch if it doesn't help with
performance, or something else.

BR.
Haiyue

> -corey
>
>> Signed-off-by: Haiyue Wang <haiyue.wang@...ux.intel.com>
>> ---
>>   drivers/char/ipmi/kcs_bmc.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/char/ipmi/kcs_bmc.c b/drivers/char/ipmi/kcs_bmc.c
>> index fbfc05e..dc19c0d 100644
>> --- a/drivers/char/ipmi/kcs_bmc.c
>> +++ b/drivers/char/ipmi/kcs_bmc.c
>> @@ -435,6 +435,7 @@ static const struct file_operations kcs_bmc_fops = {
>>   struct kcs_bmc *kcs_bmc_alloc(struct device *dev, int sizeof_priv, 
>> u32 channel)
>>   {
>>       struct kcs_bmc *kcs_bmc;
>> +    void *buf;
>>         kcs_bmc = devm_kzalloc(dev, sizeof(*kcs_bmc) + sizeof_priv, 
>> GFP_KERNEL);
>>       if (!kcs_bmc)
>> @@ -448,11 +449,12 @@ struct kcs_bmc *kcs_bmc_alloc(struct device 
>> *dev, int sizeof_priv, u32 channel)
>>       mutex_init(&kcs_bmc->mutex);
>>       init_waitqueue_head(&kcs_bmc->queue);
>>   -    kcs_bmc->data_in = devm_kmalloc(dev, KCS_MSG_BUFSIZ, GFP_KERNEL);
>> -    kcs_bmc->data_out = devm_kmalloc(dev, KCS_MSG_BUFSIZ, GFP_KERNEL);
>> -    kcs_bmc->kbuffer = devm_kmalloc(dev, KCS_MSG_BUFSIZ, GFP_KERNEL);
>> -    if (!kcs_bmc->data_in || !kcs_bmc->data_out || !kcs_bmc->kbuffer)
>> +    buf = devm_kmalloc_array(dev, 3, KCS_MSG_BUFSIZ, GFP_KERNEL);
>> +    if (!buf)
>>           return NULL;
>> +    kcs_bmc->data_in  = buf;
>> +    kcs_bmc->data_out = buf + KCS_MSG_BUFSIZ;
>> +    kcs_bmc->kbuffer  = buf + KCS_MSG_BUFSIZ * 2;
>>         kcs_bmc->miscdev.minor = MISC_DYNAMIC_MINOR;
>>       kcs_bmc->miscdev.name = dev_name(dev);
>
>