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To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        Jan Beulich <jbeulich@suse.com>
References: <20210827123206.15429-1-jgross@suse.com>
 <8b13525b-0075-f4c1-8900-1be151e85e3e@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH] xen/balloon: use a kernel thread instead a workqueue
Message-ID: <9d1edc4e-b2b5-c620-17c6-31d238c2c417@suse.com>
Date:   Wed, 8 Sep 2021 17:11:36 +0200
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From: Juergen Gross <jgross@suse.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc: Stefano Stabellini <sstabellini@kernel.org>,
 Jan Beulich <jbeulich@suse.com>
Message-ID: <9d1edc4e-b2b5-c620-17c6-31d238c2c417@suse.com>
Subject: Re: [PATCH] xen/balloon: use a kernel thread instead a workqueue
References: <20210827123206.15429-1-jgross@suse.com>
 <8b13525b-0075-f4c1-8900-1be151e85e3e@oracle.com>
In-Reply-To: <8b13525b-0075-f4c1-8900-1be151e85e3e@oracle.com>

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On 08.09.21 16:47, Boris Ostrovsky wrote:
>=20
> On 8/27/21 8:32 AM, Juergen Gross wrote:
>> +static bool balloon_thread_cond(enum bp_state state, long credit)
>> +{
>> +	if (state !=3D BP_EAGAIN)
>> +		credit =3D 0;
>> +
>> +	return current_credit() !=3D credit || kthread_should_stop();
>> +}
>> +
>> +/*
>> + * As this is a kthread it is guaranteed to run as a single instance =
only.
>>    * We may of course race updates of the target counts (which are pro=
tected
>>    * by the balloon lock), or with changes to the Xen hard limit, but =
we will
>>    * recover from these in time.
>>    */
>> -static void balloon_process(struct work_struct *work)
>> +static int balloon_thread(void *unused)
>>   {
>>   	enum bp_state state =3D BP_DONE;
>>   	long credit;
>> +	unsigned long timeout;
>> +
>> +	set_freezable();
>> +	for (;;) {
>> +		if (state =3D=3D BP_EAGAIN)
>> +			timeout =3D balloon_stats.schedule_delay * HZ;
>> +		else
>> +			timeout =3D 3600 * HZ;
>> +		credit =3D current_credit();
>>  =20
>> +		wait_event_interruptible_timeout(balloon_thread_wq,
>> +				 balloon_thread_cond(state, credit), timeout);
>=20
>=20
> Given that wait_event_interruptible_timeout() is a bunch of nested macr=
os do we need to worry here about overly aggressive compiler optimizing o=
ut 'credit =3D current_credit()'?

I don't think so. current_credit() is reading from balloon_stats, which
is a global variable. So the compiler shouldn't assume the contents
won't change.

But I can add a barrier() after 'credit =3D current_credit()' in case
you'd feel uneasy without it.


Juergen

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