Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] xhci: fix unmatched num_trbs_free
To:     Ikjoon Jang <ikjn@chromium.org>, linux-usb@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        linux-kernel@vger.kernel.org
References: <20210708164256.1.Ib344a977b52486ec81b60f9820338f1b43655f8d@changeid>
From:   Mathias Nyman <mathias.nyman@linux.intel.com>
Autocrypt: addr=mathias.nyman@linux.intel.com; prefer-encrypt=mutual; keydata=
 mQINBFMB0ccBEADd+nZnZrFDsIjQtclVz6OsqFOQ6k0nQdveiDNeBuwyFYykkBpaGekoHZ6f
 lH4ogPZzQ+pzoJEMlRGXc881BIggKMCMH86fYJGfZKWdfpg9O6mqSxyEuvBHKe9eZCBKPvoC
 L2iwygtO8TcXXSCynvXSeZrOwqAlwnxWNRm4J2ikDck5S5R+Qie0ZLJIfaId1hELofWfuhy+
 tOK0plFR0HgVVp8O7zWYT2ewNcgAzQrRbzidA3LNRfkL7jrzyAxDapuejuK8TMrFQT/wW53e
 uegnXcRJaibJD84RUJt+mJrn5BvZ0MYfyDSc1yHVO+aZcpNr+71yZBQVgVEI/AuEQ0+p9wpt
 O9Wt4zO2KT/R5lq2lSz1MYMJrtfFRKkqC6PsDSB4lGSgl91XbibK5poxrIouVO2g9Jabg04T
 MIPpVUlPme3mkYHLZUsboemRQp5/pxV4HTFR0xNBCmsidBICHOYAepCzNmfLhfo1EW2Uf+t4
 L8IowAaoURKdgcR2ydUXjhACVEA/Ldtp3ftF4hTQ46Qhba/p4MUFtDAQ5yeA5vQVuspiwsqB
 BoL/298+V119JzM998d70Z1clqTc8fiGMXyVnFv92QKShDKyXpiisQn2rrJVWeXEIVoldh6+
 J8M3vTwzetnvIKpoQdSFJ2qxOdQ8iYRtz36WYl7hhT3/hwkHuQARAQABtCdNYXRoaWFzIE55
 bWFuIDxtYXRoaWFzLm55bWFuQGdtYWlsLmNvbT6JAjsEEwECACUCGwMGCwkIBwMCBhUIAgkK
 CwQWAgMBAh4BAheABQJTAeo1AhkBAAoJEFiDn/uYk8VJOdIP/jhA+RpIZ7rdUHFIYkHEKzHw
 tkwrJczGA5TyLgQaI8YTCTPSvdNHU9Rj19mkjhUO/9MKvwfoT2RFYqhkrtk0K92STDaBNXTL
 JIi4IHBqjXOyJ/dPADU0xiRVtCHWkBgjEgR7Wihr7McSdVpgupsaXhbZjXXgtR/N7PE0Wltz
 hAL2GAnMuIeJyXhIdIMLb+uyoydPCzKdH6znfu6Ox76XfGWBCqLBbvqPXvk4oH03jcdt+8UG
 2nfSeti/To9ANRZIlSKGjddCGMa3xzjtTx9ryf1Xr0MnY5PeyNLexpgHp93sc1BKxKKtYaT0
 lR6p0QEKeaZ70623oB7Sa2Ts4IytqUVxkQKRkJVWeQiPJ/dZYTK5uo15GaVwufuF8VTwnMkC
 4l5X+NUYNAH1U1bpRtlT40aoLEUhWKAyVdowxW4yGCP3nL5E69tZQQgsag+OnxBa6f88j63u
 wxmOJGNXcwCerkCb+wUPwJzChSifFYmuV5l89LKHgSbv0WHSN9OLkuhJO+I9fsCNvro1Y7dT
 U/yq4aSVzjaqPT3yrnQkzVDxrYT54FLWO1ssFKAOlcfeWzqrT9QNcHIzHMQYf5c03Kyq3yMI
 Xi91hkw2uc/GuA2CZ8dUD3BZhUT1dm0igE9NViE1M7F5lHQONEr7MOCg1hcrkngY62V6vh0f
 RcDeV0ISwlZWuQINBFMB0ccBEACXKmWvojkaG+kh/yipMmqZTrCozsLeGitxJzo5hq9ev31N
 2XpPGx4AGhpccbco63SygpVN2bOd0W62fJJoxGohtf/g0uVtRSuK43OTstoBPqyY/35+VnAV
 oA5cnfvtdx5kQPIL6LRcxmYKgN4/3+A7ejIxbOrjWFmbWCC+SgX6mzHHBrV0OMki8R+NnrNa
 NkUmMmosi7jBSKdoi9VqDqgQTJF/GftvmaZHqgmVJDWNrCv7UiorhesfIWPt1O/AIk9luxlE
 dHwkx5zkWa9CGYvV6LfP9BznendEoO3qYZ9IcUlW727Le80Q1oh69QnHoI8pODDBBTJvEq1h
 bOWcPm/DsNmDD8Rwr/msRmRyIoxjasFi5WkM/K/pzujICKeUcNGNsDsEDJC5TCmRO/TlvCvm
 0X+vdfEJRZV6Z+QFBflK1asUz9QHFre5csG8MyVZkwTR9yUiKi3KiqQdaEu+LuDD2CGF5t68
 xEl66Y6mwfyiISkkm3ETA4E8rVZP1rZQBBm83c5kJEDvs0A4zrhKIPTcI1smK+TWbyVyrZ/a
 mGYDrZzpF2N8DfuNSqOQkLHIOL3vuOyx3HPzS05lY3p+IIVmnPOEdZhMsNDIGmVorFyRWa4K
 uYjBP/W3E5p9e6TvDSDzqhLoY1RHfAIadM3I8kEx5wqco67VIgbIHHB9DbRcxQARAQABiQIf
 BBgBAgAJBQJTAdHHAhsMAAoJEFiDn/uYk8VJb7AQAK56tgX8V1Wa6RmZDmZ8dmBC7W8nsMRz
 PcKWiDSMIvTJT5bygMy1lf7gbHXm7fqezRtSfXAXr/OJqSA8LB2LWfThLyuuCvrdNsQNrI+3
 D+hjHJjhW/4185y3EdmwwHcelixPg0X9EF+lHCltV/w29Pv3PiGDkoKxJrnOpnU6jrwiBebz
 eAYBfpSEvrCm4CR4hf+T6MdCs64UzZnNt0nxL8mLCCAGmq1iks9M4bZk+LG36QjCKGh8PDXz
 9OsnJmCggptClgjTa7pO6040OW76pcVrP2rZrkjo/Ld/gvSc7yMO/m9sIYxLIsR2NDxMNpmE
 q/H7WO+2bRG0vMmsndxpEYS4WnuhKutoTA/goBEhtHu1fg5KC+WYXp9wZyTfeNPrL0L8F3N1
 BCEYefp2JSZ/a355X6r2ROGSRgIIeYjAiSMgGAZMPEVsdvKsYw6BH17hDRzltNyIj5S0dIhb
 Gjynb3sXforM/GVbr4mnuxTdLXQYlj2EJ4O4f0tkLlADT7podzKSlSuZsLi2D+ohKxtP3U/r
 42i8PBnX2oAV0UIkYk7Oel/3hr0+BP666SnTls9RJuoXc7R5XQVsomqXID6GmjwFQR5Wh/RE
 IJtkiDAsk37cfZ9d1kZ2gCQryTV9lmflSOB6AFZkOLuEVSC5qW8M/s6IGDfYXN12YJaZPptJ fiD/
Message-ID: <e13a1d38-cc0a-51b4-fe19-7a9145e75b1d@linux.intel.com>
Date:   Fri, 16 Jul 2021 15:56:35 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <20210708164256.1.Ib344a977b52486ec81b60f9820338f1b43655f8d@changeid>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 8.7.2021 11.43, Ikjoon Jang wrote:
> When unlinked urbs are queued to the cancelled td list, many tds
> might be located after hw dequeue pointer and just marked as no-op
> but not reclaimed to num_trbs_free. This bias can leads to unnecessary
> ring expansions and leaks in atomic pool.

Good point, in that case trbs turned no-op never get added to free trb count.

> 
> To prevent this bias, this patch counts free TRBs every time xhci moves
> dequeue pointer. This patch utilizes existing
> update_ring_for_set_deq_completion() function, renamed it to move_deq().
> 
> When it walks through to the new dequeue pointer, it also counts
> free TRBs manually. This patch adds a fast path for the most cases
> where the new dequeue pointer is still in the current segment.
> 

This looks like an option.

Another approach would be to keep the normal case fast, and the special case code simple.
Something like:

finish_td()
...
	/* Update ring dequeue pointer */
	if (ep_ring->dequeue == td->first_trb) {
		ep_ring->dequeue = td->last_trb;
        	ep_ring->deq_seg = td->last_trb_seg;
        	ep_ring->num_trbs_free += td->num_trbs - 1;
        	inc_deq(xhci, ep_ring);
	} else {
		move_deq(...);
	}

move_deq(...)
{
	while(ring->dequeue != new_dequeue)
		inc_deq(ring);
	inc_deq(ring);
}

inc_deq() increases the num_trbs_free count.

I haven't looked at the details of this yet, but I'm away for the next two weeks so
I wanted to share this first anyway.

-Mathias