Re: [dpdk-dev] [PATCH] cast used->idx to volatile

[Linhaifeng <haifeng.lin@huawei.com>]

On 2015/3/24 18:06, Xie, Huawei wrote:
> On 3/24/2015 3:44 PM, Linhaifeng wrote:
>>
>> On 2015/3/24 9:53, Xie, Huawei wrote:
>>> On 3/24/2015 9:00 AM, Linhaifeng wrote:
>>>> On 2015/3/23 20:54, Xie, Huawei wrote:
>>>>>> -----Original Message-----
>>>>>> From: Linhaifeng [mailto:haifeng.lin@huawei.com]
>>>>>> Sent: Monday, March 23, 2015 8:24 PM
>>>>>> To: dev@dpdk.org
>>>>>> Cc: Ouyang, Changchun; Xie, Huawei
>>>>>> Subject: Re: [dpdk-dev] [PATCH] cast used->idx to volatile
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 2015/3/21 16:07, linhaifeng wrote:
>>>>>>> From: Linhaifeng <haifeng.lin@huawei.com>
>>>>>>>
>>>>>>> Same as rte_vhost_enqueue_burst we should cast used->idx
>>>>>>> to volatile before notify guest.
>>>>>>>
>>>>>>> Signed-off-by: Linhaifeng <haifeng.lin@huawei.com>
>>>>>>> ---
>>>>>>>  lib/librte_vhost/vhost_rxtx.c | 2 +-
>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
>>>>>>> index 535c7a1..8d674d1 100644
>>>>>>> --- a/lib/librte_vhost/vhost_rxtx.c
>>>>>>> +++ b/lib/librte_vhost/vhost_rxtx.c
>>>>>>> @@ -722,7 +722,7 @@ rte_vhost_dequeue_burst(struct virtio_net *dev,
>>>>>> uint16_t queue_id,
>>>>>>>  	}
>>>>>>>
>>>>>>>  	rte_compiler_barrier();
>>>>>>> -	vq->used->idx += entry_success;
>>>>>>> +	*(volatile uint16_t *)&vq->used->idx += entry_success;
>>>>> Haifeng:
>>>>> We have compiler barrier before and an external function call behind, so we don't need volatile  here.
>>>>> Do you meet issue?
>>>>>
>>>> Tx_q is sometimes stopped when we use virtio_net. Because vhost thought there are no buffers in tx_q and virtio_net
>>>> though vhost haven't handle all packets so we have to restart VM to restore work.
>>>>
>>>> The status in VM is:
>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246687] net eth7: virtnet_poll
>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246690] net eth7: receive_buf
>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246693] net eth7: vi->num=239
>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246695] net eth7: svq:avail->idx=52939 used->idx=52939 num_free=18 num_added=0 svq->last_used_idx=52820
>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246699] net eth7: rvq:avail->idx=36215 used->idx=35977 num_free=18 num_added=0 rvq->last_used_idx=35977
>>>> Mar 18 17:11:11 linux-b2ij kernel: [46337.901038] net eth7: dev_queue_xmit, qdisc->flags=4, qdisc->state deactiveed=0
>>>> Mar 18 17:11:12 linux-b2ij kernel: [46337.901042] net eth7: dev_queue_xmit, txq->state=1, stopped=1
>>>>
>>>> Why compiler barrier not take effect in our case? Is compiler barrier depended on -O3 option? We use -O2 option.
>>> compiler barrier always works regardless of the optimization option.
>>> I don't get your story, but the key thing is, do you check the asm code?
>>> If called from outside as an API, how is it possible it is optimized?
>>> there is only one update to used->idx in that function.
>>
>> Do you mean rte_vhost_enqueue_burst also not need cast used->idx to volatile ? Why not remove it?
> I checked the code. Seems we can remove. That is another issue.
> For your issue, you meet problem, and submit this this patch, but i am a
> bit confused it is the root cause. Do you check the asm code that
> volatile is optimized?
> 

I had wrote a demo try to find out the different between rte_compiler_barrier and volatile.
It seems no any effect on rte_compiler_barrier().

-------->test1: without rte_compiler_barrier and volatile

#include <rte_atomic.h>

int main()
{
        int i,j;

        *(int*)&i = 2;
        *(int*)&j = 3;
        printf("i=%d j=%d", i, j);
}
linux-LOubNs:/mnt/sdc/linhf/test # gcc -S test.c -I /usr/include/dpdk-1.7.0/x86_64-native-linuxapp-gcc/include/ -O3
linux-LOubNs:/mnt/sdc/linhf/test # cat test.s |grep main -B 10
        .file   "test.c"
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "i=%d j=%d"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
.LFB571:
        movl    $3, %edx
        movl    $2, %esi
        movl    $.LC0, %edi
        xorl    %eax, %eax
        jmp     printf
.LFE571:
        .size   main, .-main
		
		
-------->test2: use rte_compiler_barrier
note: the asm code same as test1
		
linux-LOubNs:/mnt/sdc/linhf/test # cat test.c
#include <stdio.h>
#include <rte_atomic.h>

int main()
{
        int i,j;

        *(int*)&i = 2;
        rte_compiler_barrier();
        *(int*)&j = 3;
        printf("i=%d j=%d", i, j);
}
linux-LOubNs:/mnt/sdc/linhf/test # gcc -S test.c -I /usr/include/dpdk-1.7.0/x86_64-native-linuxapp-gcc/include/ -O3
linux-LOubNs:/mnt/sdc/linhf/test # cat test.s |grep main -B 10
        .file   "test.c"
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "i=%d j=%d"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
.LFB571:
        movl    $3, %edx
        movl    $2, %esi
        movl    $.LC0, %edi
        xorl    %eax, %eax
        jmp     printf
.LFE571:
        .size   main, .-main

		
-------->test3: use volatile
		
linux-LOubNs:/mnt/sdc/linhf/test # cat test.c
#include <stdio.h>
#include <rte_atomic.h>

int main()
{
        int i,j;

        *(volatile int*)&i = 2;
        *(volatile int*)&j = 3;
        printf("i=%d j=%d", i, j);
}
linux-LOubNs:/mnt/sdc/linhf/test # gcc -S test.c -I /usr/include/dpdk-1.7.0/x86_64-native-linuxapp-gcc/include/ -O3
linux-LOubNs:/mnt/sdc/linhf/test # cat test.s |grep main -B 10
        .file   "test.c"
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "i=%d j=%d"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
.LFB571:
        movl    $2, -4(%rsp)
        movl    $3, -8(%rsp)
        movl    $.LC0, %edi
        movl    -8(%rsp), %edx
        movl    -4(%rsp), %esi
        xorl    %eax, %eax
        jmp     printf
.LFE571:
        .size   main, .-main

>>>>>>>  	/* Kick guest if required. */
>>>>>>>  	if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
>>>>>>>  		eventfd_write((int)vq->callfd, 1);
>>>>>>>
>>>>>> --
>>>>>> Regards,
>>>>>> Haifeng
>>>>>
>>>
>>> .
>>>
> 
> 
> .
>