linux-kernel - Re: [PATCH 1/3] virtio-crypto: fixup potential cpu stall when free unused bufs

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Message-ID: <91aa6ce0-e8a3-21ac-d29e-b2a47f6386d1@linux.alibaba.com>
Date:   Fri, 9 Jun 2023 21:49:39 +0800
From:   Xianting Tian <xianting.tian@...ux.alibaba.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     arei.gonglei@...wei.com, mst@...hat.com, jasowang@...hat.com,
        xuanzhuo@...ux.alibaba.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, amit@...nel.org, arnd@...db.de,
        marcel@...tmann.org, johan.hedberg@...il.com, luiz.dentz@...il.com,
        linux-bluetooth@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xianting Tian <tianxianting.txt@...baba-inc.com>
Subject: Re: [PATCH 1/3] virtio-crypto: fixup potential cpu stall when free
 unused bufs


在 2023/6/9 下午9:41, Greg KH 写道:
> On Fri, Jun 09, 2023 at 03:39:24PM +0200, Greg KH wrote:
>> On Fri, Jun 09, 2023 at 09:18:15PM +0800, Xianting Tian wrote:
>>> From: Xianting Tian <tianxianting.txt@...baba-inc.com>
>>>
>>> Cpu stall issue may happen if device is configured with multi queues
>>> and large queue depth, so fix it.
>>>
>>> Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>
>>> ---
>>>   drivers/crypto/virtio/virtio_crypto_core.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/drivers/crypto/virtio/virtio_crypto_core.c b/drivers/crypto/virtio/virtio_crypto_core.c
>>> index 1198bd306365..94849fa3bd74 100644
>>> --- a/drivers/crypto/virtio/virtio_crypto_core.c
>>> +++ b/drivers/crypto/virtio/virtio_crypto_core.c
>>> @@ -480,6 +480,7 @@ static void virtcrypto_free_unused_reqs(struct virtio_crypto *vcrypto)
>>>   			kfree(vc_req->req_data);
>>>   			kfree(vc_req->sgs);
>>>   		}
>>> +		cond_resched();
>> that's not "fixing a stall", it is "call the scheduler because we are
>> taking too long".  The CPU isn't stalled at all, just busy.
>>
>> Are you sure this isn't just a bug in the code?  Why is this code taking
>> so long that you have to force the scheduler to run?  This is almost
>> always a sign that something else needs to be fixed instead.
> And same comment on the other 2 patches, please fix this properly.
>
> Also, this is a tight loop that is just freeing memory, why is it taking
> so long?  Why do you want it to take longer (which is what you are doing
> here), ideally it would be faster, not slower, so you are now slowing
> down the system overall with this patchset, right?

yes, it is the similar fix with one for virtio-net

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/net/virtio_net.c?h=v6.4-rc5&id=f8bb5104394560e29017c25bcade4c6b7aabd108 
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/net/virtio_net.c?h=v6.4-rc5&id=f8bb5104394560e29017c25bcade4c6b7aabd108>

>
> thanks,
>
> greg k-h