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Thu, 19 Feb 2026 06:44:31 -0800 (PST) X-Received: by 2002:a05:600c:214b:b0:47e:e051:79ee with SMTP id 5b1f17b1804b1-4839fe90522mr29037115e9.3.1771512270783; Thu, 19 Feb 2026 06:44:30 -0800 (PST) Received: from [192.168.0.56] ([78.16.129.5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483a31b1d79sm21327665e9.3.2026.02.19.06.44.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Feb 2026 06:44:29 -0800 (PST) Message-ID: Date: Thu, 19 Feb 2026 14:44:28 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] net/mlx5: check for no data read in devx interrupt To: Slava Ovsiienko , Stephen Hemminger Cc: "dev@dpdk.org" , "NBU-Contact-Thomas Monjalon (EXTERNAL)" , "david.marchand@redhat.com" , Dariusz Sosnowski , "stable@dpdk.org" , Harman Kalra References: <20260128122055.192104-1-ktraynor@redhat.com> <20260206172054.273858-1-ktraynor@redhat.com> <20260206172054.273858-2-ktraynor@redhat.com> <20260206220939.5a90c67c@phoenix.local> From: Kevin Traynor Autocrypt: addr=ktraynor@redhat.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org On 10/02/2026 20:58, Slava Ovsiienko wrote: > Hi, > > What about checking EPOLLERR | EPOLLHUP | EPOLLRDHU flags for specific fd in mlx5 habdler? > > if devx_get_async_cmd_comp() returns EAGAIN { > if no data were read { > call epoll_wait() for specific fd and zero timeout > check EPOLLERR | EPOLLHUP | EPOLLRDHU flags > if fd is in hanging/error state { > - remove handler > } > } > } > Thanks for the suggestion. We already have this info at the time of callback. So I added an API to get it and made it os agnostic in v4. Let me know if you have comments or suggestion. Thanks! > With best regards, > Slava > >> -----Original Message----- >> From: Kevin Traynor >> Sent: Tuesday, February 10, 2026 9:08 PM >> To: Slava Ovsiienko ; Stephen Hemminger >> >> Cc: dev@dpdk.org; NBU-Contact-Thomas Monjalon (EXTERNAL) >> ; david.marchand@redhat.com; Dariusz Sosnowski >> ; stable@dpdk.org; Harman Kalra >> >> Subject: Re: [PATCH v2 1/2] net/mlx5: check for no data read in devx interrupt >> >> On 10/02/2026 17:05, Slava Ovsiienko wrote: >>> Hi, >>> >> >> Hi Slava, >> >>> I'm sorry, I have some concern about the patch. >>> >> >> No problem, that's what reviews are for :-) thanks for reviewing. >> >>> How it works, as far as I understand: >>> >>> - DPDK simulates interrupts in user mode with epoll_wait() >>> - mlx5 PMD emits the async counter query command to the NIC >>> periodically >> >> I didn't think this would happen unless there was something like hardware >> offload, but regardless, yes I agree there may be async counter queries. >> >>> - there might be multiple async query commands in the flight >>> - kernel drivers handles the async query completion interrupts, pushes >>> the token to the internal completion queue and unblocks associated fd >>> - epoll_wait() sees this unblocked fd and notifies mlx5 PMD about >>> - mlx5 PMD reads the completion token from the kernel queue with >>> devx_get_async_cmd_comp() >>> >>> The concern scenario, let's assume: >>> >>> - we have 2 async query commands in the flight >>> - the first async query completes, fd is unblocked, PMD is inviked, >>> the completion is read by PMD and is being handled >>> - the second async query completes, fd gets unblocked, the second >>> token is written to the queue >>> - the PMD completes the handling of the first completion and reads the >>> queue again (with devx_get_async_cmd_comp() call in the loop) >>> - it reads the second token successfully and handles >>> - then, on the third call, devx_get_async_cmd_comp() returns EAGAIN, >>> it means queue is empty >>> - DPDK calls epoll_wait() again and sees unblocked fd >>> - it call mlx5 PMD, and it calls devx_get_async_cmd_comp(), but queue >>> is empty (handled in previous interrupt handling) >>> - with the patch we wrongly remove the handler >>> >> >> I'm not sure, but this ^^^ sounds feasible. >> >>> In my opinion, we should handle flags EPOLLERR | EPOLLHUP | EPOLLRDHU >>> from the epoll_wait()_return also for RTE_INTR_HANDLE_EXT and >> RTE_INTR_HANDLE_DEV_EVENT interrupt types. >>> >> >> That's exactly what I had in v1 of the patch! The issue is that some clients of eal >> interrupt may not interpret the condition of EPOLLHUP/EPOLLRDHUP as an >> error condition and/or want to do some special handling. >> >> The example is vhost user server, which puts in place a reconnect mechanism. If >> we filter out EPOLLHUP/EPOLLRDHUP events in eal, then virtio will not receive >> the callback and vhost server reconnect is broken. I have some more notes >> about it in the cover letter. >> >> Trying to base on the read pattern in devx handler was an attempt to move logic >> out of eal so different handlers could be flexible in how they handle this >> condition. >> >> We do have a distinction in that mlx5 uses RTE_INTR_HANDLE_EXT and virtio >> uses RTE_INTR_HANDLE_VDEV but i'm not sure that is generic enough to base a >> check/don't check for EPOLLHUP/EPOLLRDHUP events on. >> >> So we'd need to come up with another solution if we wanted to filter this in eal. >> Let's think more on this, though we are a bit constrained by public API as well. >> >> A workaround we can do from application is David's hack™ "-a 0000:00:00.0" >> to skip initial probe. That will at least prevent the issue for mlx devices not used >> in DPDK, which was the scenario reported. >> >> thanks, >> Kevin. >> >>> With best regards, >>> Slava >>> >>> >>> >>> >>>> -----Original Message----- >>>> From: Kevin Traynor >>>> Sent: Tuesday, February 10, 2026 5:06 PM >>>> To: Stephen Hemminger >>>> Cc: dev@dpdk.org; NBU-Contact-Thomas Monjalon (EXTERNAL) >>>> ; david.marchand@redhat.com; Dariusz Sosnowski >>>> ; Slava Ovsiienko ; >>>> stable@dpdk.org >>>> Subject: Re: [PATCH v2 1/2] net/mlx5: check for no data read in devx >>>> interrupt >>>> >>>> On 07/02/2026 06:09, Stephen Hemminger wrote: >>>>> On Fri, 6 Feb 2026 17:20:53 +0000 >>>>> Kevin Traynor wrote: >>>>> >>>>>> A busy-loop may occur when there are EPOLLERR, EPOLLHUP or >>>> EPOLLRDHUP >>>>>> epoll events for the devx interrupt fd. >>>>>> >>>>>> This may happen if the interrupt fd is deleted, if the device is >>>>>> unbound from mlx5_core kernel driver or if the device is removed by >>>>>> the mlx5 kernel driver as part of LAG setup. >>>>>> >>>>>> When that occurs, there is no data to be read and in the devx >>>>>> interrupt handler an EAGAIN is returned on the first call to >>>>>> devx_get_async_cmd_comp, but this is not checked. >>>>>> >>>>>> As the interrupt is not removed or condition reset, it causes an >>>>>> interrupt processing busy-loop, which leads to the dpdk-intr thread >>>>>> going to 100% CPU. >>>>>> >>>>>> e.g. >>>>>> epoll_wait >>>>>> (6, [{events=EPOLLIN|EPOLLRDHUP, data={u32=28, u64=28}}], 8, -1) >>>>>> = >>>>>> 1 read(28, 0x7f1f5c7fc2f0, 40) >>>>>> = -1 EAGAIN (Resource temporarily unavailable) epoll_wait >>>>>> (6, [{events=EPOLLIN|EPOLLRDHUP, data={u32=28, u64=28}}], 8, -1) >>>>>> = >>>>>> 1 read(28, 0x7f1f5c7fc2f0, 40) >>>>>> = -1 EAGAIN (Resource temporarily unavailable) >>>>>> >>>>>> Add a check for an EAGAIN return from devx_get_async_cmd_comp on >>>>>> the first read. If that happens, unregister the callback to prevent looping. >>>>>> >>>>>> Bugzilla ID: 1873 >>>>>> Fixes: f15db67df09c ("net/mlx5: accelerate DV flow counter query") >>>>>> Cc: stable@dpdk.org >>>>>> >>>>>> Signed-off-by: Kevin Traynor >>>>> >>>>> AI spotted this, I didn't... >>>>> >>>>> >>>>> Errors: >>>>> >>>>> Line 139: Unnecessary semicolon after closing brace >>>>> >>>>> c >>>>> >>>>> }; >>>>> >>>>> Should be: >>>>> c >>>>> >>>>> } >>>>> >>>>> Lines 142-146: Block comment uses incorrect style Block comments >>>>> in C >>>> code should use /* and */ style, not /** which is reserved for >>>> documentation comments. >>>>> >>>>> c >>>>> >>>>> /** >>>>> * no data and EAGAIN indicate there is an error or >>>>> * disconnect state. Unregister callback to prevent >>>>> * interrupt busy-looping. >>>>> */ >>>>> >>>>> Should be: >>>>> c >>>>> >>>>> /* >>>>> * no data and EAGAIN indicate there is an error or >>>>> * disconnect state. Unregister callback to prevent >>>>> * interrupt busy-looping. >>>>> */ >>>>> >>>>> Warnings: >>>>> >>>>> Logic clarity: The variable data_read is set to true inside the >>>>> while loop but >>>> never checked when data WAS read. Consider if data_read is the >>>> clearest way to express this condition. >>>>> >>>> >>>> Ack above. Thanks.Will be fixed in v3. >>> >